As for the Nord Stream, Russia has much more convenient way to send a message because it controls the valves. On the other hand, to stop the Nord Stream, the US(or NATO) would need to pressure Germany to stop receiving LNG from Russia or blow up the pipes.

If the NATO wants to blow up the Nord Stream pipes(as in the conspiracy theory from your post), it would not use Russian bombs. When you conduct underwater military operation such as this, you don't use enemy's bombs. You would send an expertly trained team with equipment and devices you can trust.

Find a couple of Russian bombs installed at Baltic pipes before detonation, disengage them, move them to the Nord Stream, reinstall them, and then detonate? To me, that is highly unlikely.

Biden openly declared he would stop the Nord Stream if Russia invaded Ukraine, although the pipelines were under German control. He didn't elaborate on how and just said something like "I promise you, we'll do it". Victoria Nuland of State Department also said basically the same thing. She also didn't elaborate on how. I think they were speaking against Russia knowing that Russia would never stop or sabotage its own pipelines.

Selling LNG to Europe for a big money can't be a sole reason to do it. But it shows that blowing up the Nord Stream pipes is not so bad for the US. It's bad for Russia because it lost a significant revenue source and useful mean to pressure Germany or Europe. But ultimately, I think it's really bad for Europeans(common people, not necessarily the elites).