I understand you are laying out a smorgasbord of options available to Russians:

.. adopt a defensive military stance, because they CAN defend themselves militarily ..

Surely this would be very dangerous for Russia. For one, within (say) few years the US will develop hypersonics (if North Korea can etc.) and it would be a few minutes flight for missiles to reach Moscow (say). Also swarming of missiles may be the tactics adopted by NATO, so something is likely to get through. Why risk it, with the only other option being WW3?

.. but the west probably CANNOT defend itself economically.

What does west get from Russia?: hydrocarbons i.e. oil and gas, possibly wheat, some metals. The markets will re-adjust, still plenty of oil and gas from ME in the short term. As for Europe, it will suffer for a spell and US will be ready to sell their fracking gas until they build new pipelines. Bit expensive but hey …

Russia will be stymied in trading with the rest of the world since Russian carriers may then be curtailed from flying over territories that are controlled by the west, which is not very efficient i.e. economic, and the main trading partner for Russia would then be only China. Etc.

Just testing your theory ..