I see zero gain for Russia to blow up their own pipelines. And where they went bang the surrounding countries will know who have vessels in the area and would have conformed that it was Russia by now.
It's not easy blowing a pipeline up (at that point it would be 27.8mm wall thickness) and mines/charges could have been placed at any time since it was laid, it had to be sophisticated ordnance. Possible that a hole/s were drilled first and then the charges intiated.
The offshore section of NS1 was 9Billion Euros and NS2 11Billion. Why would they waste those assets? Makes no sense at all to me, there's no gain politically either.
I worked on NS1 and a mate of mine who also did was approached at the airport by MI5 and asked if he'd be willing to give them info on it so from day one our security services were looking at it and the Amercicans were dead against both pipelines from the off.
They wouldn't have happened without massive backhanders to get the permits and most importantly the full on support of Germany.... they're rueing that now!