The Sevmorput has more than enough power to drop her anchor and drag it along the bottom

An act of war.

It was Batman

How does a ship underway even at low speed deploy anything capable of locating and damaging an undersea cable and a pipeline? I would be more inclined to think their arrival was coincidence and that the explosives had been placed there hours or days before the blast occurred, probably by rovs deployed remotely.

Locating? You don't think Russia and China know EXACTLY where these things are... Seems pretty clear they did.

It's called an anchor....super sized cargo carrier like these. Can drop and drag anchors...destroy cables and pipes

NewNew Polar Bear is a 16,000 ton feeder container cargo ship, the second of four Eilbeck-class feeder container ships commissioned by the German shipowner Hansa Hamburg Shipping International in the early 2000s. She is designed to work smaller ports in the Baltic. She was originally named MV Reinbeck and until earlier this year owned and operated by European companies. It's a tenth the size of the container cargo ships crossing the Pacific and Atlantic.
My vote for the culprit goes to the Russian ship as it is a LASH and has a 500 ton gantry that can drop things off the stern.
Think about motive for a minute. There is no reason for China to damage European infrastructure, nothing to gain and a great deal to lose, but Russia is another matter altogether.

Who gains the most benefits from the sabotage,,,,and who can take the most risk the least repercussions

USA and UK 😎