So anybody with Magic skill can become a Wizard?
Yup. You'll also need the appropriate Wizard spells, though, which only a wizard, or an import, would have, or you won't be able to succeed.
He's not entirely true on this one - yeah, anyone with magic skill can become a wizard and, if your thieving skills are up to task, you can do all the robberies available, but that special event for EOF joining just never happens if you're not a fighter.
Don't think I've ever seen that event, fighter or otherwise, but the game does tend to be pretty hectic, so perhaps I missed it. You sure it's based solely on being a fighter, and not merely having asked about the right stuff and the possession and use of a sword?
Endgame also varies a bit depending on your starting class, but it still is passable through the use of off-class skills, you can even combine them a bit.
Are you certain it's based on your class, and not the specific path or skills you have? I rechecked a walkthrough of it, and the "Thief" ending seems much more familiar to me, even though I don't recall playing as an official thief, but always have thief skills.
Sorry to disappoint you, but the paladin's previous profession makes no difference, meaning that he gets the shield, he starts using it in combination with the sword and he gets paladin-specific events instead of the wizard & thief specific ones
None of those events you describe are part of QFG2, though. The class-specific lockins don't really start until 3, where your entry class actually shuts out options. Overall, though, the series definitely gets more class-centric as it goes on: In 1, your class didn't matter at all, not in the slightest, except insofar as it determined your abilities and equipment, in 2, it seems mostly the same way, but by 3, it is definitely starting to block you in.