There are plenty of calls you can debate over this just isn't one if we go by the rules. It's certainly unlucky as Tadic was in the wrong position in the wrong time.
100% debatable. Again, this isn't a rule that says offside or not. Passive offside is interpretative and requires interfering with the opponent. Are you purposefully ignoring that no one is arguing that he wasn't offside? You said offside is not up for interpretation, when in this type of offside it 100% is. The ref and VAR determined through
INTERPRETATION that he interfered with Courtois while in an offside position.
As I said though, it was disallowed, water under the bridge now. Having seen multiple times now, I think it was a 50/50 call on the interference, so fine that ref decided that (and it's fine that you agree), would have been fine the other way too. Same deal with the Benatia penalty we conceded last year too. Once immediate aftermath passions calmed, it was a 50/50 call imo.