I missed one vital piece of information. There is no way for Russia to tell whether it is being attacked by conventional warheads or nuclear.
As such, Russia has to respond as if they are nuclear. And, then I cannot see any other outcome after than than a nuclear war.
So, If the US fires Tomahawks at Russia from Ukraine (and it is the US firing them), then we must assume WWIII is starting.
The Russians have no red lines. They have no lines that if they are crossed necessitate action, and they have no red lines that have to be crossed before there is action. They will start WWIII when it is advantageous to them to start it rather than not. That has nothing at all to do with any red line being crossed.
Nor will they respond to Tomahawks being fired using the defence they plan for a nuclear attack, because that reveals their hand. Nor will they attack in a limited “conventional” way with nuclear ready weapons, for that allows the unproven US “defence” system to be tested. Nor will they allow the Tomahawks to be a success.
If you let loose a herd of raging buffalo, and the only way they can go is through the kindergarten, then whether the Kindergarten is a few meters or miles away, if that is the only path than can be followed, the raging buffaloes will kill the children, whether it seems directly connected or not.
Once the Russians are tipped into a nuclear war scenario, even by one nuclear ready missile, they have very little option but to play to win completely. And, the best way to win completely is an attack totally out of the blue.
The PR
Yes the Russians are currently doing all they can to avoid civilian casualties and indeed probably unnecessary governmental and military casualties. But, once WWIII starts, the Russians will no longer want to play the “victim” of US aggression. For they couldn’t care less about what anyone thinks of them if they are forced to play the nuclear hand. They will not be looking for any brownie points, just victory at all costs.
All that nice guy stuff about avoiding civilian casualties goes out the window as soon as they are pushed down the nuclear option. After WWIII the winner will writing the history books any way. The loser will be just another centrist Socialist …. sorry National Socialist aka “Hitler” whoever wins.