The knock-on was from Parra to the ground. The player who first picked up the ball reacted fast enough and was flat enough that you'd have a hard time calling it deliberately off-side (he probably thought he was behind the ball when it was knocked on). It's marginally offside at best, and not deliberately, and there were no Stade Francais players that could have realistically scored a try immediately, so scrum five to Stade is the sensible call here.
As a referee, I have used a push or tug on the shoulder to de-escalate a situation. I started as an ice hockey referee and linesman, so learned some useful techniques to do it safely (linesmen are expected to break up fights and pull apart combatants). When I started refereeing rugby, I was able to transfer those skills. »
Except as referees, we are taught that a series of infringements in "the red zone" can be grounds for carding, regardless of repetition of a given offence or the same player infringing multiple times. If you feel there is a pattern of cynically breaking the laws in order to stop a team from scoring, you can certainly card someone...typically it's the next bloke that infringes after you have a stern chat with the captain.