In both cases the team awarded the penalty has the player in the bin, so of course they can decide not to take a quick tap. That will allow the referee to bring the player back on and speak to him if necessary before play is resumed.There is a useful clarification in Law:
[LAWS]Clarification 2 2007
Ruling in Law by the Designated Members of the Rugby Committee
Ruling2-2007
Union / HP Ref ManagerIRFU
Law Reference3
Date12 March 2007
Request
1. A penalty kick is awarded to White team. Before the kick is taken, can White team make a substitution? (If so, we assume that the substitute can take the penalty kick.)
2. A penalty kick is awarded to White team. Before the kick is taken can a previously sin-binned White player return; given that the 10 minutes have fully elapsed? (If so, we assume that the returning player may take the penalty kick.)
Ruling in Law by the Designated Members of the Rugby Committee
A substitution can only be made when the ball is dead. A referee must not let a player rejoin the match until the ball is dead. The ball is dead when the ball is out of play. This happens when the ball has gone outside the playing area and remained there, or when the referee has blown the whistle to indicate a stoppage in play, or when a conversion kick has been taken.
In both cases outlined above, the ball is dead, and therefore the answer to each question is in the affirmative.[/LAWS]
At a PK, the ball is dead, the player may return. So if 80 min has gone, but there is a PK, then the player may return.
That clarification does not apply if the penalty is awarded to the opposition, who may well decide to take a quick tap.