It doesn't add up tot me, don't only mean incentives of Russia, but mainly how i understand your post: NS2 was accidentily leaking due to bad workmanship, but it was deliberately done in the deepest part for ease of repair.

Also for an underwater pipe, wouldnt the part of the pipeline with a hole flood up the rest of the pipeline until the point where you can stop the (water) flow by a valve or by maintaining high enough gas pressure