I was simply floating an idea I had seen in another sport, in this case, Ice Hockey
Their sin bin is called The Penalty Box.. and it literally is a box...
Their Penalty Box is two minutes. If a team scores a goal while an opponent is serving a
non-coincidental minor penalty, the penalty is cancelled and the player may return to the ice.
That "non-coincidental" thing answers your other question. If the penalty is incurred in the scoring of a goal, that goal doesn't bring the player back, the next one does. In reality the Ice Hockey rule is a bit more complicated than that when it come to double (more serious) penalties, I can explain it if you wish but I wasn't thinking of using it here anyway.
What I had in mind was the preventing of the situation we had last week where France copped a YC (an unjustified one IMO) and while the player was off, NZ scored three converted tries.