Bang-bang-thud obstruction question

[Paul L]

Fed rules. B1 hits slow grounder to F6, who fields it and throws to F3. F3 has foot on first, but leaving access for B1. Throw comes in low and offline toward the home plate side. F3 comes off the bag about a foot, bends down, and catches ball. As she bends down, her hips move slightly backward, as is natural when bending over (try touching your toes with your heels touching a wall). The hip movement unintentionally blocks the basepath. B1, without first slowing or altering her path (no time to react), glances off F3's hip and falls to foul ground past first, without touching the bag. The time from F3 beginning the bend though the contact to the catch was no more than a quarter-second. F3 purposely steps on first base just after B1 hits the ground. Ball hit glove an instant after runner hits fielder.
1: Obstruction?
2: Safe at first for passing the bag without touching it and wait for a definite appeal?
3: Nothing/collision/train wreck/incidental contact?
4: If ball hit glove instant before runner/fielder contact and fielder dropped it, interference?
5: If ball hit glove before player contact, but after B1 veered or slowed, what then?
6: When is a collision a nothing?