I'm sorry I don't have my books to give references but in general NFHS "loose ball plays" are legal forward pass, scrimmage kick and free kick plays. All other plays are running plays. A fumble on a running play does not change the enforcement spot from end of run(running plays) to previous spot(loose ball plays). That's why be bag the spot of a fumble, to mark the end of the run, that becomes the enforcement spot. (I'm speaking in generalities, not covering all possible exceptions)

1st and 10 from A40. A10 drops back, scrambles, runs to A45 then throws a complete forward pass to A80 to B40.

If this was a "loose ball play" then enforcement of IFP would be the previous spot or A40. But this is a running play therefore enforcement is from A45. (end of run) Just like Sit b

Fumbles, IFP and backward passes do not constitute "loose ball plays" only loose balls.

Only if the spot of the fumble is beyond the NZ.

Sit b) was behind the NZ.

Fumbles and backward passes can be loose ball plays, if they occur behind the NZ. According to 2-33-1c:

Art 1 . . . A loose-ball play is action during:
A backward pass (including the snap), an illegal kick or fumble made by A from in or behind the neutral zone prior to a change of team possession.

In sit b), it was a fumble behind the neutral zone prior to a change of possession, thus it was a loose ball play. In my situation, I was specifically targeting a loose ball behind the NZ, thus the previous spot enforcement.

I got some clarification on B at my meeting last night. Since A fumbled behind the LOS the Basic Spot is the Previous Spot, not the end of the run like I said earlier.

There's also no need to bag the fumble in this situation since it is not an enforcement spot.