

For good reason the US and others don’t make public their playbook for responding to nuclear attacks on themselves or others with very few exceptions. We know about “mutually assured destruction” in the case of an ICBM, but not much else.
“Proportional” probably doesn’t apply in the case of nukes, it’s usually described as a maximal response to ensure it never happens again. However, the rumor has always been that an enemy using a nuke on an allied country would result in the US engaging in “total eradication” of that government. Probably, in this case, the complete destruction of Moscow. If I had to guess I’d think that would (since we’re pretty damn sure we could) do that with conventional arms to limit escalation.
They only get out on parole if the board (or whatever the UK’s equivalent is) believes they have learned a lesson and are safe to reenter society. If not then they stay until the next board meeting. I have a hard time imagining they would get out after 20 years for murder without the board being pretty damn sure they are no longer a danger to society.
As gross and heinous as these crimes are, I will never favor life-without-parole or the death penalty for minors.