First question: who could have blown the pipelines?
Semi-covertly:
Any navy with a deep sea submarine fleet (US, UK, France, Russia), plus
Any regional navy with any submarine fleet (Norway, Sweden, Finland, Poland, Germany)
I say semi-covertly here because the Baltic is a strategic post of the world and undersea microphone arrays are relatively cheap that close to land. I would expect that the militaries of the US, Germany, Poland (and possibly all of NATO), and Russia knew the class and possibly name of every surface ship and submarine that was anywhere near the pipelines in the last two weeks.
Overtly:
All of the above plus
Every other Baltic nation (Denmark, Latvia, Estonia, Lithuania)
Any of a dozen of so major subsea construction companies
Any nation with a deep sea navy
I tend to discount the last one because it would have been moderately big news if a navy ship from China or India or Australia was rolling around the Baltic. Since that wasn’t in the news, it probably wasn’t them.
I don’t think the subsea construction theory is very plausible either. Those companies are very large, very risk-averse, and they depend on being able to work in a lot of places. They definitely wouldn’t spend a few million of their own money to blow a pipeline. Even if they were being paid by another entity, they would be very likely to be found out and blackballed from working in moderately large parts of the world, not to mention the likelihood of criminal charges. That’s almost certainly a higher risk than they’d be willing to pay.