Not looking for opinions here - looking for rule support. In order to not poison the well, I won't offer my thoughts or research. But here's a hint, whatever we decide is what I ruled.
B1 intercepts a pass in his endzone. While trying to run it out of the endzone, A1 accidentally grabs B1's facemask (5 yard variety). After the facemask foul, B1 fumbles the ball and it rolls out of bounds at the 2 yard line.
Where is the enforcement spot?
Related question. What is the basic spot (and rule reference) for fouls that occur after change of possession?
This is research for discussion at a meeting, so please don't offer opinions. Please cite the rule book (or casebook or official interpretations). When we're done, I suggest you bring this to a meeting - leads to interesting discussion. I understand this is the subject of a long discussion of opinions on another board, but I brought it here 'cause I'm always impressed by the knowledge of those who hang out here.