In my opinion, for VAR in general, it should just work like this:
If the referee sees an incident, his call on the field stands. So if someone commits what looks like a foul in the box and the ref says I saw it but i dont consider it a free kick, its no free kick. VAR cant overule him. Hes the ref. There will always be some subjectivity at the end of the day.
VAR can only support the ref. So VAR can check something obvious that the Ref may not have seen and advise the ref. If VAR has new information that the ref missed he can check it out on the monitor. VAR is like a linesman, theyre just giving advice to the ref.
With regards to offsides I think there should be something similar to cricket, where there is an umpires decision or in this case linesmans decision.
There is a small zone, like in cricket, by which the linesmans decision is the ultimate arbitrar of whether something is offside. After all, VAR is only supposed to step in with clear and obvious errors. A toe being offside, in the heat of the moment when a player running is not a clear error of offside from the linesman.
So for example you could make this a 5cm or 10cm area, a zone of doubt, beyond the offside line drawn by VAR. If the attacker is in this area they are still onside.
Now you might think well what if youre right on 5cm beyond the VAR line - youre just shifting the line. Well the thing is youd have no arguments that VAR called you offside incorrectly. Because even if VAR draws the line slightly incorrectly by 1-2cm youre still 3cm offside anyway.
Youre basically giving an attacker the benefit of the doubt (which offiside is supposed to be), and if its right on the margins, well the attacker cant complain if it goes against them because they were beyond the offside line anyway.
This would sort out the arguments about the exact line of where VAR draws offside, and also when VAR is allowed to step in for incidents.