Not enough liquidity in the market.

Technically, do the defacto principles of no first strike and deterrence through retaliation mean that Russia has no reason not to nuke both the Ukrainian forces in Kursk and Ukraine itself?

Striking in Ukraine itself could be the retaliation plan for any arbitrary further red line. Striking both areas would leave nothing exceptional to threat with.

True, it would leave nothing exceptional to threaten with to deter against further transgressions, but it would also leave no enemy to commit said transgressions

In case it wasn't clear, I hope they don't, but I fear that they have no selfish reason not to now

I am 80% sure that if Ukraine loses and putin is still alive, then within 2 years putin attacks Estonia. He will make new enemies.