Its a real tough question this. Firstly if retrospective bans were used not sure of the benefit to the fouled team. If someone like Sane gets injured a player being bans for 3 matches against say Chelsea, United our rivals is hardly a compensation. That lends itself to VAR for me, if the correct decision is not given, reviewed upstairs and a signal given to the ref to send the offender off. Next time the ball out of play he goes.
The refs need help it is quite simple. I watch matches all the time on tv and it is only when the slo-mos are shown you fully realise the height of the challenge/lateness of the tackle/studs showing etc. In the majority of cases I doubt the ref fully realises the severity.
Also as much as we blame the refs when do the players take some responsibility? Not just fouls, diving, exaggerating contact, shirt pulling etc. Easy to blame the refs and avoid the more difficult issues of commentating on managers and players who are the real culprits.