NFHS Rules:

Does change of possession matter when a foul occurs on the opponents of the scoring team?

Such as A2 is holding on a running play? Then A1 fumbles. B recovers and returns it for a touchdown.

I couldn't find anything in the rule book or case book stating that B will have to decline A penalty even if it occured before a change of possession.

Yes, it does matter. The foul must be after the change of possession.

8-2-2 is printed correctly. Per the NFHS site, it should read:

8-2-2 Correction – Replace with: If during a touchdown-scoring play, a foul by the opponents of the scoring team occurs on a play where there is no change of possession; or a foul by the opponents of the scoring team occurs after the change of possession, if there is a change of possession, the scoring team may accept the results of the play and have the penalty enforced from the succeeding spot.

go to this site: http://www.nfhs.org/scriptcontent/va...ot=FB_FOOT.cfm

There's some good stuff here.

Oops. I wanted to say:
8-2-2 is printed incorrectly.

Canadian Ruling
 

B will just decline the fouls as though it never happened.

And then count 6 points.