Neverwinter Nights 2/WMG

Deekin is an epic level Harper Agent.
He sells ridiculously good prestige class equipment to the protagonist. The gear was actually from the Harper Agent special inventory. Deekin's role was to ensure the defeat of the King Of Shadows. He obviously didn't take the Red Dragon Disciple path in Hordes of the Underdark, so he'd be unable to defeat the King of Shadows himself. So, he operates from the sidelines to level up the protagonist. First, he pulled some strings to prevent the protagonist from getting into the noble's area too soon. Later on, Deekin made sure that the protagonist would be made Knight-Captain of Crossroads Keep.
 * But if he's allowed to level with no guidance, by default he will take the Red Dragon Disciple Path. But, only the KC would be able to use the Silver Sword of Gith to destroy the Portal, even Deekin would have a hard time dealing out enough damage to destroy it otherwise, so all he can do is help you along with his Harper connections.

Most of the party survived.
As confirmed by Mask of the Betrayer, Jerro and Khelgar both survived. Elanee can Wild Shape into an Earth Elemental and earth-glide out. One of the arcane casters died, but if Sand survived the initial rockfall he could teleport out. Neeshka has Improved Evasion, there is no way falling rocks could hit her. The Construct is literally Made of Iron, it'll be fine. Casavir and Zhjaeve I'm not sure about, but I think a quick Miracle from Zhjaeve could carry everyone to safety. As for Grobnar, who cares?
 * He got an awesome death. We don't want that undone, do we?
 * Sorry to bust this one. Jerro definitely survives (you can meet him in the expansion). Khelgar also survives (he goes to your wedding!) Elanee and Casavir both die (boo hoo! Both of your romances are dead!) Sand also survives (he turns himself into a golem). The construct attempts to save Grobnar but both die. Zhjaeve's fate is the only ambigous one, as she simply disappears. And we all know what happened to Bishop, don't we?
 * The ending for romance in MoTB notes that Nesska, Khelgar and Sand survived, (meaning it was Qara who died, but seing as how Sand is alive, the PC or a companion killed her, because she always defects if Sand stays, no matter how high her influence) are alive, while gameplay shows Ammon Jerro is.
 * Ammon Jerro confirms Casivir is dead, broke his back holding something so everyone else could pass IIRC. Elanee is dead, don't recall how.
 * Elanee's not dead. The game says she left the party after the encounter at the Circle of the Mere, which does happen in the OC if you don't pass influence check.
 * Where are you getting all this? I might of missed something, but I got the impression that the only confirmed things were that Khelgar survived the initial rockfall (The imps mention he yelled at them when they were fetching you) and that Jerro escaped, but the Pit Fiends caught up to him and took his soul; which is destroyed opening the door to the Headmistress' study. Really though, there is so many ways that the party could have escaped being buried deep underground that it isn't a huge surprise that only Bishop and Qara, (Who you kill) would die. Admittedly, I could have missed some of this because I chose the ending where you stay in the Upper Plains to prevent Akkaj from returning and if I hadn't done that the ending would have mentioned some of this or possibly I screwed something up by not returning Jerro's soul to his body. This does make you wonder why Lord Nasher never tried resurrecting you, finding a level 17+ cleric can't be very hard in the Forgotten Realms.
 * Nope...Khelgar didn't die. As it's said above, he goes to your wedding, but you only see this if you . Lord Nasher never tried to resurrect you because OC ending stated that they only found your cloak, which makes sense because you got abducted. Several characters did die in the rock falls, also stated above, but you only know about this if you talk to Ammon, which looks like you didn't. Oh, and pit fiend did not remove Ammon's soul......you only know about this if you talk to Ammon, too.
 * Just to clarify things, when speaking with Ammon in the expansion pack, you get different dialogue choices that identify what happened in your game. These affect what he tells you about the events. If Sand stayed with you, he says that he heard him casting a shapechange spell, meaning that he might have survived. If Neeshka stayed with you, Ammon says that she managed to make it out. If Elanee stayed with you, he implies that she dies (last saw her trapped under a rock, not moving). Ditto for Qara and (though you don't find this out until later) Bishop. Khelgar apparently always survives, while Casavir, Grobnar, and the Construct are always killed in the collapse. That just leaves the fate of Zhjaeve undetermined.
 * There seems to have been a bug in my game, then. Elanee stayed with me until the bitter end, but Ammon said she left me after the Circle meeting.
 * No bug, just a case of Damn You, Muscle Memory!. Among the dialogue choices on the screen before he starts telling you who lived and who died is one that lets you tell him that Elanee left after the Circle meeting, not the other way around. (It's the same set of choices where you choose whether Qara or Sand stayed loyal.) I'm guessing you clicked all the available options before clicking the one that moves to the next set of options (the one where Ammon starts telling you what he saw).
 * As an aside -- Zhjaeve is a Githzerai, and Gith of Zhjaeve's level can shift between planes at will. There's a good chance she got out, and that no-one saw it happen.
 * Update: In Storms of Zehir, a character mentions that the Luskans found a paladin in the ruins and captured him for suspected involvement in Sydnee Natale's death (you kill her in the OC). So it seems Casavir may not be dead after all.
 * Well really, how many paladins do YOU know of that could be stuck in the ruins? I think it's pretty clear he's still alive. Although, considering what we know of Luskan and how they treat supposed 'criminals', he might not be for very much longer.
 * The only reasons to believe anyone at all died is the original ending, which is shown to be fallible, and Ammon Jerro's testimony, and we all know what a fount of coherent and truthful information he is. It's much more likely that the entire high-level party just ignored something as insignificant as falling boulders.
 * Especially since the character that can planeshift at will (Zhjaeve), the shape-shifting Druid with an Earth Elemental form (Elanee if you didn't lose her) AND an artifact the team was carrying (that anyone can use) can cast resurrection spells.
 * Qara is probably still dead, if Sand is alive then the bitch betrayed you and you killed her for it. Grobnar's probably dead too, because he had an awesome death. And Bishop is definitely dead because you saw him absorbed into The Wall.

Light of Heaven is a PC.
The guy playing her is pretty new to the whole "role playing" concept.
 * Elaborate?

Kelemvor wants the next crusade to succeed.
He's all about justice and even handedness, and the wall's pretty much the unliving embodiment of Disproportionate Retribution. He can't simply tear it down because directly countermanding a god's decree makes the afterlife that much more uncertain, something that Kelemvor want to avoid at costs. So, when the latest Spirit Eater comes calling on the Fugue Plane, rather than just teleporting him/her to the Wall and saying "there you are", he makes it as big a spectacle as he can, inspiring further attempts to imitate, and giving them a victory.
 * This does make sense. I mean, if he was going to offer you to just take your soul and go, why wait until after you've run amonk all across his ciy. Besides, he just states that you piss off the gods of order, as in all the Lawfull good and neutral gods. And even some of them might be on your side, like Hoar.
 * Heck, Kaelin the Dove seems to be a Canon Immigrant in 4E, so he's not making too great an effort to stop her raids.
 * The wall apparently doesn't exist in 4E either. It gets no mention in the new setting supplements, even the descriptions of that plane.
 * If he doesn't defend the wall, doesn't that mean that Asmodeus gets to eat all the Souls that would go to it, like he could before the Wall was set up, and then he'll destroy the Gods and make the 4E FR setting even more of a Crapsack world? I was under the impression that was the whole story behind the wall to begin with.
 * Not entirely. Mrykul mostly made the thing so he'd have somewhere to put unclaimed souls. Demons still try to steal souls from it, and Devils often buy them, so to speak. It's in Kelemvor's realm, and since it's divinely morphic, he could just take the wall out with a thought. That said, it's missing in 4e, so it's entirely possible your PC comes back later and destroys it, he says you're not strong enough to fight him in his realm, not yet, or that Kaylen eventually took it down.

Zhjaeve's apparent lack of a deity wasn't a mistake.

 * The reason is Zhjaeve is listed on her character screen as faithless is not that she gets her powers from a Gith god, but she has found a way to replicate divine powers through psionics. After all, the Githzerai and Githyanki are extremely powerful in mind magic. Pity Psion or Wilder wasn't a class option.
 * In the default D&D setting a cleric can serve a cause rather than a specific god.
 * Not in the Forgotten Realms they can't.
 * She clearly identifies herself as a Zerth, meaning she worships the Githzerai hero and founder Zerthimon (who no-one else in D&d does). In most supplements this is the default for Githzerai priests, since they consider conventional worship a form of slavery.
 * It's not officially listed in the rules, but Githzerai are supposed to be, lore wise, an exception to the rules related to deity selection.

Casavir is the PC from the first game.

 * He left Neverwinter for reasons of his own after something personal with a woman, and his leaving was considered a betrayal by many. We know from the Underdark expansion that the NWN PC was a male human sword-swinger who left Neverwinter in disgust after Nasher executed Aribeth. Also, the statue to the Hero Of Neverwinter in the park looks a bit (but just a bit) like him, especially in the default equipment he has when you get him.
 * But he's rather low level for that. I usually came out a 16-18th level, and even then I usually capped up all the way to 20.
 * Level != in-universe power. Still a reasonable guess, but I doubt it's what the developers intended.
 * Actually level is a measure of in-universe power. However, level 16, contrary to popular belief, isn't "low." Level 20 is supposed to be theoretically the absolute highest possible attainable by a mere mortal. Level 20+ is effectively weak demigod. The emphasis on epic levels heavily colored this.
 * Definitely not the intention, a quest that was cut due to time constraints explained why Casavir left Neverwinter. He was in love with Ophala and killed her other suitor.

The Rocks Fall, Everyone Dies ending was deliberate.

 * Near the end of his Let's Play of the game, Lt. Danger comments that the game has a number of heroes along with the PC. All of them failed at heroism, whether in a fall to villainy (the King of Shadows, once the Guardian; Ammon Jerro, the warlock blinded by his desire to defeat the King of Shadows; Lorne the old fighting champ of West Harbor), some inherent personal flaw (Daerred the imbecile; Cormick, the other West Harbor champ who went from champion to City Watch "middle management") or an ignominious death (Shandra Jerro the would-be PC). He then asks "Why does the PC not fail?"
 * But the PC does fail. He died ignominously under a massive pile of rocks in his moment of nominal glory. And of those who cheered and booed him in Neverwinter, how many knew him by title ("Knight Captain", "Squire of Neverwinter"), birth ("Harborman"), or accident of circumstance ("Kalach-Cha")? And how many knew him by name? Only the ones who presumably died with him under that same pile of rocks. In a BioWare title like Mass Effect, heroism ends in victory, leaving the PC's mark on the galaxy for generations to come. In Neverwinter Nights 2, heroism ends in an enemy you can't stop--death by lair collapse.
 * The PC doesn't die in the ending. Only two characters  actually die in the ending, and it was probably to make sure they never, ever had to appear in the expansions, for fairly obvious reasons.  also die, but they usually die before the ending.

Kelemvor is a Magnificent Bastard.

 * He's not allowed to undo Myrkul's work, but he has no problem letting a mortal end Akachi's curse permanently.
 * Kaelin's raids on the wall in the epilogue? You'd think if he wanted to he could banish her from the Fugue Plane for good, or just use his godly powers to kill her. Again, he allows a mortal to do what he is forbidden.
 * So how does this theory explain the But Thou Must! lack of an option to destroy the Wall for good?
 * The PC can't really try, even in a hopeless fight, so that's not exactly Kelemvor's fault. Also, he may have reasons for wanting the Wall to remain for the time being, including the possibility that allowing the crusaders to try might draw the attention of other deities that they wouldn't be able to defeat (and that he can't afford to directly oppose) yet, even if he wants it gone eventually. And even if he doesn't want the Wall to be destroyed, he could have other reasons for allowing the crusades - mortals repeatedly demonstrating that they can be a threat to the will of gods, even a minor one, could both motivate mortals to do great things and help keep some gods from getting too big for their britches.

Grobnar wasn't killed by the falling rocks...
He was saved at the last moment by the Wendersnaven. Come on: in your heart, you know it to be true.
 * Gods, I hope not.