The thing is that at that point almost everyone would benefit from it (maybe everyone except Europeans). Russia because they could put pressure on Europe by cutting gas supply during winter, US because they wanted to convert Europe to LNG and put pressure on the gas revenues of Russia.
So my conclusion is that an international task force did it lol
On the other hand blowing up the pipe means giving up on :
1. blackmailing the europeans if they don't comply to some demands. 2. A huge revenue - because BTW there's no way it can be repaired.
So I don't think contractual ramifications weighs much in that context
But from historical perspective, the one thing that outlives wars, is Debt and Contracts. Specially if they are owned by the winner.
In any case, Russia could 'turn the gas off' by having 'irrepairable turbine failures' or various other technicalities.