1. I dunno, good question!

2. Yes, Biab uses the same recorded instruments, just differently
Realband allows features BiaB doesn't; rendering multiple takes of the same RT, recording multiple audio tracks, etc
BiaB makes things faster for the initial effort; like finding chords and styles, but RB offers more expansion.
They are similar but very different.

3 & 4 didn't really have questions, so they are hard to answer.

I do not work here, but the benefits are still awesome
Make your sound your own!