Q –  RP 14:2

Kickoff returner is tackled by someone not on the field – awarded TD – does the entire return yardage get credited to the returner?

A – You can never be awarded a touchdown via penalty.  So, if in the opinion of the officials, the returner would have scored if not for the illegal substitution and subsequent tackle, and he is awarded a touchdown, he is credited with all the return yardage . . . and the player coming off the bench would most likely be disqualified for a flagrant and dangerous action.