N-R, the way you mention that the perpetrator is not there is perhaps a bit too generous in determining if it is to be a PT. If it was a 1 on 1, and the defender put in a high tackle, if he was not there then it would be a run in unopposed, and that is probably not going to meet the law requirement of "but for the foul play would have scored in a better place". So in this instance, think what would have happened if the perpetrator was there but didn't put in a high tackle. I think that is the measure for deciding on a PT.
Re the issue with the collapsed maul, if there was no grounding then there was no try to award, so the only option really would be to award a PT as the foul play had prevented the try, so good refereeing here.