diver,
Don't be so quick to rule out the Russians. They still have gas export capability, and may have calculated that, in the short term at least, manipulating gas prices would be more lucrative. They could have done it much more easily than anyone else. And they could have been attempting to frame someone.

'Tis unlikely to be the Germans, but I wouldn't rule them out completely. They need the gas, but they do have other sources of it.

Rule out the Chinese, it's way outside their area of operation and they'd have far more to lose than to gain.

>So who dun-it, the US or Ukraine?
Why would you regard those countries as the prime suspects?
It clearly wasn't Ukraine; they don't have the capability. But even with your hasty ruleouts, why did yo not consider Britain or Sweden?
Putin blames Britain.
Sweden had submarines active in the area. If they didn't do it, they almost certainly know who did.

I still think Russia's the prime suspect, but it's far from certain.