1. If characters are blocking all squares of a doorway, can a flyer still pass through/over them?

2. If friendly characters (represented by ABC) and non-friendly characters (represented by XYZ) are in a doorway/congested area:

A |
BC|
XYZ

can figure A, who is grounded, move through figure C and then move adjacent to Z, stopping, or is he prevented from doing this because moving through C would already make him adjacent? Or is this too confusing?

Yep.

Moving through C would make him adjacent so it is not allowed.

thx