You could do this whole plan without needing to do the assassination part, as long as you are willing to throw Ukraine under the bus. Conversely, even if the assassination scheme goes exactly as planned, there is no way of guaranteeing that the new Russian leader would be willing to restart gas deliveries until the war in Ukraine has wrapped up. Given that especially the eastern EU countries have absolutely no intention of allowing Ukraine to lose, this seems like a very tall order.
Finally, both Nordstream 1 AND Nordstream 2 still have a gaping holes in them from the bombings so restarting deliveries will probably take several years at least.
All in all, this plan gets only a 2 out of 10 for being impractical, too slow AND depending on factors outside our control. 1 point because it does at least sound spy-ish and proactive.