> My theory is the Russians blew up NS1 / NS2 to cause pain to the EU over the winter and gain an advantage to show that EU needs them.

Neither NS1 nor NS2 were in use; the only scenario that really makes sense for Russia is that they new Europe wasn’t reversing course on Ukraine without some additional threat, and thought that blowing up NS1 & 2 would make an effective threat against Baltic Pipe.

The “Ukrainian-friendly non-state entity did it to foreclose from European states the option of resuming gas purchases from Russia” idea is not (in motivationbterns) implausible.