Straddling the pivot foot

[wyo96]

Had this a few times over the past few games in both MS and F/JV Boys. Hard to describe but I will try. I also know it is somewhat a HTBT.

B1 and B2 have full court press. As A1 crossed midcourt, picks up dribble. B1 & B2 close in tight. As a result B1 is straddling A1's pivot foot. (A1 has stretched out to pivot around).

By rule, is the resulting contact the responsibility of A1 or B1?
Each player is entitled to their legally obtain space on the floor, but does A1 space extend out when he stretches out?

If B1 does not move forward for a push, can he still occupy the space?
If A1 wants to pivot and trips on B1 legs is it travel, block, charge?

I feel good about the calls I have made/not made when this happens, but am looking for some insight and possibly some clarification.

Most of the time there is a clear choice, but once in a while it is more murky.