No matter who actually did it. Russia would blame Ukraine/NATO/USA/The West in general, together and separately in rotation.
After that I have no real idea, who would succeed. There would be a massive power struggle both in the government and military.
There is a strong possibility that several of the autonomous regions within the Russian federation would try to break away. There is about a 20% chance of a civil war because of this.
There would be a power vacuum, probably a few murders/suicides/defenstrations, and the elections scheduled for a few months from now will probably be postponed during the struggle for leadership of the country. My guess is the powers that eventually take over will agree to pull out of Ukraine, conduct a few show trials, and do whatever else they think they have to in order for the west to lift the sanctions and unlock the oligarch's accounts.
It would please me greatly to know that Ukraine will outlive Putin.
That'd probably be true for a while, but after that they'd need to start an "election" where some military commander or other higher up will take his chair.
destabilization until the Kremlin finds a new president in that time probably multiple nations would be indipendent and Ukraine would pretty much launch their offensive and the union state collapses and tv stations all across Russia hijacked
A hardliner will take power (Putin is not the most antiwest russian politician) and then...from the total erasing of Ukraine to third world war, anything would be posible, but neither civil war nor colapse of Rusia. That is the OTAN countries wet dream.
This is very likely. If anything, the existing system will be perpetuated because there's no replacement atm. It's been too long in the making and it will take a lot of work to undo. Also, he may be long dead and it won't change anything in the near future.
This said there's always a chance for a change, but not right now.