First point - most of it is now taken off the table because there is an implicit (explicit?) unknown party with the proven ability and willingness to cut off any remaining supply at the drop of a hat, and the ability to supply is severely curtailed. It stops 'cheating' or 'backsliding' on the part of either party.
Second point - agreed. If for no other reason than there is little to no incentive for any of the players to share any evidence or info they may have found that would support or disprove any of the scenarios.
For Russia, if they could prove the US did it, it would strengthens the image of the US as a powerful world player with their foot on Russia's neck. If someone else did it, it would make them look even weaker.
For Western European allies, it would make it really obvious how much influence the US has on them, especially since their own fate continues to depend on the US - and it's large natural gas supplies. Even if they wanted to cut off the US, Russia is even worse for them, and they can't stand on their own two feet against either Russia or the US right now (militarily or economically). If someone other than the US did it, it would make their key infrastructure look even more fragile and vulnerable.
For the US, if they did it, it would expose the extent they are playing dirty (hurting the 'clean hands' narrative) and lose them good will with most of the public. If they found someone else doing it, it would reduce their apparent 'dirty tricks' power folks need to worry about, which is a major deterrent to enemies and allies doing dirty tricks.