A few thoughts on this subject, though not directly answering the OP's questions:
3. Over the years, I've seen a lot of emphasis on backstories, which is good. But I'm more interested in what the characters' goals are. In my current campaign, we have a drow rogue who wants to build her own hideout, ala the Continental from the John Wick stories. There's also a storm cleric who wants to build a temple to his god on an island, so he can convert the inhabitants. Those things give me useful hooks for future adventures, and help tie the characters to the ongoing campaign. Backstories only get you so far - at some point, they tend to get resolved (avenged my fallen family, unlocked the secret scrolls, etc.).
Croesus said this much better than I did

Back story is great, but mostly as a driver for motivation. Try to hook into that, and keep in tune with how that motivation may change over time.