Referring to rule 4-12-3b (2001-02 Rule books) for definition of control:
"Team control continues until the opponent secures control."
Despite Team A fumbling the ball all over, and off Team B, Team A retains team control and is therefore still liable for a BC violaiton...
per Rule 9-9-1
if they are the last to touch in the FC (and thereby cause it to go into the BC) and first to touch in the BC.
I think overall, as officials we miss the correct call on these situations way too often. I know I have missed a couple this season but I'm getting better.