The spot of first touching is NEVER a spot of enforcement. The only time that particular place on the field becomes a spot of enforcement is after the down is over, and everything associated with that down, has been taken care of. At that point, that spot can become a succeeding spot. Then, if there's a foul after that, it can be marked off from that succeeding spot. In the case of the live-ball-treated-as-a-dead ball, while it carries a succeeding spot enforcement, it's still part of the continuity of the down in which it happened. Therefore, you can't use the spot of first touching if the penalty is accepted. You would have to go to the succeeding spot, which IS THE SPOT WHERE THE BALL WOULD BE PUT INTO PLAY HAD THERE BEEN NO FOUL.
Now, you can argue that if there had been no foul, the ball would have (technically could have) been put into play at the spot of first touching, and this is true. However, doing so would put you in violation of the "right of first touching by R goes away if the penalty is accepted for any foul committed during the play." There is no exception to that fundamental. It doesn't say, "unless there is a live ball foul treated as a dead ball foul. In that case you can use the spot of first touching."