No surprises about Putin's new position (see post to which I am responding). It's exactly as predicted, except the mobilisation of 300k Russians is also designed to increase the cost to Ukraine of continuing to fight.
I'm sure Biden has long since relayed to the Russians the rules under which they are operating. Go nuclear and we will respond by various means that are so scary you will regret what you have done.
I know Britain has done something similar. We launched nuclear subs with the instruction that if Moscow launches nuclear strikes on Britain, they will obliterate Russia.
This is all just Mutually Assured Destruction. Same as it ever was. The threats are a weapon to scare the general public.
The main rule of MAD is, don't launch a missile, idiot.