Excellent questions! Unfortunately, Delver can be tough to explain. I think the easiest way to try to grasp it is to listen to this session I recorded with a dear friend for 1e.
As for the differences between 1e and 2e:
- new cover art! From the incredible Sasha Reneau.
- character growth is part of the game instead of an optional appendix
- new examples for play
- information reshuffled and rewritten to make the game easier to follow - this is the biggest set of changes, and too comprehensive to list individually
If you have any other questions, I'm happy to answer them.