Held ball/Alt poss.

[BEAREF]

This situation is described in the case book (6.4.5 situation B). The ruling is "Since the throw -in had not ended and no violation occurred, it is still A's ball for an alternating-possession throw in. (4-42-5).

Rule 4-42-5 "The throw-in ends when the PASSED ball touches, or is touched by, another player who is either inbounds or out of bounds."

What you were told is correct

[Adam]

I believe (no rules book handy) a foul by B1 here would be a personal foul. I think if B1 reached across and fouled A1 OOB, it's an intentional foul (combined with the delay warning or T). If B1 reaches across and hits the ball, automatic T (and warning if the warning hasn't been given). Here's a hint; try to get the reach before the contact if you can.