What possible gain is there for Russia to blow up the off-ramp to the gas sanctions?

I didn't say Russia did it. I mean it probably did but Germany isn't off the table. Unlike the US Germany actually has the stealth subs to pull it off undetected, but all in all Russia is still the more likely option I'd say. Of course, the presence of ships in that area etc. is only circumstantial evidence.

And in your analysis you're making a crucial mistake, a mistake I myself made directly before the invasion when Russian soldiers were getting itchy underwear on Ukraine's border because I thought if they're going to attack, they'd already have done it: You assume Russia is a rational actor. Or, maybe better put, that it considers the same things as rational as you do.

Blowing up NS2 from Russia's side could have the motive of a) knowing or suspecting that you don't need it any more -- though it also wouldn't be terribly hard to repair which people are constantly overlooking and b) to provide an excuse to stop deliveries. Russia was playing around back at that time with NS1 maintenance and turbines being needed which were stuck in the sanctions regime etc, allthewhile Germany was filling its gas storage and nationalising Russian gas assets on German soil. They might've thought that they need to disable NS2 so Germany wouldn't say "well if NS1 doesn't work why don't we use NS2".

As to the US threats: What was probably meant was sanctions. It's true that the US has levers it can pull to force such an issue. Those would come at a cost to the US itself but they're there and can be pulled if the cost is deemed acceptable.

And btw one thing is for sure: Germany will never again buy any (noticable) amount of Russian gas. Even if they retreat to their own borders tomorrow that ship has sailed, Germany is in full swing to replace all that fossil infrastructure with ammonia and hydrogen. NS2 is dead no matter whether it's operational or not.

Oh another thing is for sure: Ukraine is way more important to Russia, or maybe better put Putin, than some gas pipeline. Pretty much the moment Germany changed laws to legalise sending weapons into crisis territories, i.e. Ukraine, Russia knew where Germany stood, and will continue to stand. We don't tend to flop easily and they know it. As such it also might simply have been Putin being stroppy, expecting Germany really to go for that Duginesque¹ division of Europe between great powers things, with Germany taking a forceful lead in Europe. He did later on comment that "siding with Ukraine was Germany's mistake of the millennium" or something to that effect. So much for Putin's rationality, he's living in a completely different world than us, thinking state relations and decisions work on fundamentally different principles than they actually do.

¹ not really, Dugin never came up with that stuff he's not a theorist he just rehashes nationalist bullshit those theories actually date back to the German Empire trolling the Russians to bait them it's a long story.