Russia doesn’t lose all leverage the moment they shut off the pipeline. They still have the leverage from being able to turn the pipeline back on, which impacts competitors and customers by giving the option.
Blowing up the pipeline takes that option off the table for the foreseeable future, and with the advantage that it doesn’t cause immediate dangerous supply shocks to Allies since it was already off.
Win/win for the Allies (though if public, Western Europe gov’ts would have no choice but to be pissed in public), not great for Russia who has their last leverage knocked off the table.
I personally don’t have an opinion on if the US did or did not do it, and I doubt we’d know for at least several decades.
But the US has done lots weirder stuff with far less concrete potential benefits before. hell, nearly anything the CIA had been caught doing in the 60’s or 70’s has far less plausible justification!
My personal belief is that we will never know who actually performed the acts of sabotage. But taking some Biden soundbites, mixing it with some public information and some hand-waving doesn't produce any actual evidence about who actually did it.
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.