Defense Offsides?

[keystoneref]

I have a question about this particular situation. Does the foul for the defensive substitute not getting to his side of the neutral zone (illegal substitution, 5 yds, simultaneous with the snap) automatically become illegal participation if this player participates in the play? Or is it a seperate foul with a second flag at the spot of participation with options for the offended team? The reason I ask is this. Game situation is this 3rd and 4 at the 50. B1 potential substitute does not get to his side of the neutral zone before the snap. A sweeps away from him and B1 doesn't participate or follow the play. A gets stuffed at the line and retreats and reverses the play back toward B1. B1 now makes the tackle 15 yards behind the line of scrimmage. If we have a seperate foul for illegal substitution simultanious with the snap, A could choose that option and have a 1st and 10 at B's 45. If we don't have a seperate penalty and you determine B1's participation occured at the spot of the tackle we now penalize 15 yards from the end of the run and we have 3rd and 4 from the 50. I am curious how everyone would handle this situation.