Tales of Phantasia/WMG

Trinicus is Mint's dad

Mint suspiciously lacks any reference to a father figure, despite both of Cress's parents being introduced and Chester's pre-existing orphan status being explained.
Trinicus is conveniently the only character in the game's Present time period other than Mint and Meryl who has any understanding of clerical artes.
When the party first meets him at his manor, Trinicus passively mentions he was friends with Cress's parents, but he tells Mint he knew her mother "very well. Perhaps even better than you did."

Arche is 13 in half-elf years

As in, she's in her thirties or more in human years. Physiologically prepubescent, but due to her half-human blood, she's aged mentally at a human rate. Hence her rather... adult personality, and not being a virgin, and so on.

Arche is 17.
Also Arche wasn't really all that "adult" (by adult I assume you mean perverted. This troper remembers a there was the scene where she peeks in on the guys in the Hot Springs, but other then that she wasn't all that perverted.) and I don't remember an mention of her not being a virgin (although she might not be) as mentioned Arche is 17. Also I think she lives with her father who didn't look too old.
- It's heavily implied that Arche is not a virgin because she didn't think she would be able to approach the unicorn. Remember that only "pure maidens" are allowed to see the unicorn.
- And that leads to other Guessing, "who took her chasteness?"
  - This one time, at magic camp...

Sometime between Symphonia and Phantasia, Undine and Celcius fused together

This would offer an explanation as to what happened to Celcius, and why she isn't seen in Phantasia.

Reasons this makes some sense:
- 1. Undine in Symphonia was a formal, proper guardian, and Celcius was a Badass warrior lady. In Phantasia, Undine is... a Badass warrior lady.
- 2. Symphonia, Celcius fought with bursts of Ice magic and physical marital arts, and Undine fought mainly with water magic, and would occasionally summon a water sword. In Phantasia, Undine fights with the sword all the time, and has added Ice magic to her repertoire, in addition to her water magic.
And anyway, if their forms are as mutable as we know them to be, why couldn't they do a Fusion Dance?
- Aw...that leaves poor Volt without a Distaff Counterpart

The Lester and Morrison families team up behind the scenes in the altered timeline.

They need to make sure no one messes up the timeline they worked so hard to create. Using Klarth and Edward's memoirs they create the seals on the mausoleum and keep an eye on things to make sure that the Heroes of Eternity can do their job without interference once they arrive in the future(s). This probably also makes Klarth responsible for the seals on the blades of Ice and Fire, since they sort of made a Stable Time Loop with the Eternal Sword... Which means you can blame him for the fight with Fen Beast.

Asshole.

The GBA version is a retelling of the legend.

Not the original story itself, but rather, someone, somewhere, at some point in the future, telling someone else of the legend of the heroes who mastered time itself to defeat Dhaos. If it's been long enough, this would explain the discrepancy in the names: people no longer remember what they were really called.

Dhaos and Cress are somehow related.

They both have blond hair, a red headband, and a red/orange cloak/cape. If they aren't actually related, they were meant to show their similarities.

Dhaos knows Kratos.

Kratos was Dhaos's mentor.