I'm not disagreeing with the outcomes of anything people have said, but I feel we're missing something in the details here because it sounds like we're creating exceptions to rules that don't exist.
An IFP *is* a loose ball play. The BASIC SPOT for a loose ball play is the previous spot, and it always is. However, under the all-but-one principle, the ENFORCEMENT SPOT may be the spot of the foul, and not the basic spot, if it is a foul by A behind the basic spot.
For an IFP behind the LOS, the basic spot is the LOS but the spot of the foul is the spot of the IFP, behind the LOS. The penalty is enforced ABO from the spot of the foul. It is not "enforced as a running play", but rather as a loose ball play under the all-but-one principle.
If there was an IFP *beyond* the LOS, then the basic spot is the end of the related run (i.e., where the IFP was thrown). Since the spot of the foul and basic spot are the same spot then the enforcement spot is one and the same. Since throwing an IFP requires the ball in player possession first, the spot of an IFP beyond the LOS must always be the spot of the end of the related run.
The "exception" that's relevant comes up if the penalty for an incomplete IFP is declined. If a forward pass is incomplete, the ball is returned to the previous spot if legal and to the spot of the pass if illegal.
Again, the net result isn't different from what other people said, but trying to think of it as some exception to the rules may lead to further confusion down the line.