Obstrcution at the plate?

[mach3]

I had a situation which broad up a lot of discussions. ISF Rules, but I guess ASA is the same:

Bases Loaded. Batter bunts and F3 fields it. Catcher stands in front of and partly on the plate for the force out. F3 however races the runner to the plate.

I am PU and only watch the feet to try to figure out which pair belongs to whom. F3 beets runner for the force out. But runner also steps on the plate.

Coach than comes out and wants obstruction against the catcher called since she was not in possession of the ball and the runner had to hesitate, which was the reason F3 was late.

Now here come the questions:
a) When does the catcher has to leave the plate area? How late is still ok?
b) How much space is enough for the runner at the plate?
c) Who should call obstruction if it was one?

Thanks

Raoul