Superman II/Headscratchers

WHY did Superman have to give up his powers at all?
Man of Steel, Woman of Kleenex

Why did Lois and Superman sleep with one another?
Superman and Lois have, for a very, very long time, held the titles of Moral Arbiters. They are examples to be held up by kids...and now they've slept with each other out of wedlock. Does this mean that it's okay for our kids to do this? No, not at all! What were the writers thinking?
 * Nothing wrong with non-self-destructive sex between two consenting adults. That and the issue of "making her an honest woman" was only a problem in societies where women would be refused marriage opportunities if they weren't virgins, which isn't the case anymore. The sexual revolution was 40 years ago, long before this movie- long enough for Superman, Lois, and the audience to have grown up in and been familiar with it. The only thing they did wrong was not using a condom and/or not making sure Lois took The Pill afterwards.
 * And even these last wouldn't be "wrong" if they intended to have children together.
 * As a side note, Lois Lane spent the better part of the years before this movie scheming and plotting to make Superman marry her, often against his will and in utterly ridiculous ways, in the comics. So much for being a moral arbiter.

Upon arriving on Earth, Ursa is bitten by a snake, which she then fries with her heat vision to prove their Superman-like powers. Must've been a Kryptonian snake to be able to bite her at all...

 * Going by the comics, the longer a Kryptonian spends under a yellow sun, the tougher they become and the stronger their other abilities are. Of course, immediately emerging from the Phantom Zone, the three of them are able to breathe and tear apart a Moon Lander, so.. *shrugs*

After he lost his powers at the fortress, how did Superman and Lois get back to America?
They didn't. The diner they stopped at on their way back was in Canada.

Superman was told that if he gave up his powers, he'd have to do so permanently. So why does he get his powers back?

 * The Richard Donner Cut answers this to some degree. Jor-El foresaw the possibility that Kal-El would want to give up his powers to be with an Earth woman. He also foresaw the possibility that Kal-El would change his mind. Since Jor-El was about to die on Krypton anyway, he imbued the green crystal with all his Kryptonian life essence, which Kal-El could use to regain any of his powers he'd given up ... once. "The son becomes the father, and the father the son."