There have been “Nuclear first-use and threats or advocacy thereof” and those are easy to condemn. But as far as I know they are coming unilaterally from the Russian side and already being widely condemned by those not on the Russian side. But it sounds like you are looking for some broader consensus to condemn escalation on both sides.
Unfortunately neither this post nor the open letter you linked give any specifics about what other behaviours you are asking us to condemn. I’m reluctant to risk endorsing a false-equivalence argument by signing a blank check.
Is blowing up the Kerch bridge escalatory? Is Arestovich trolling the occupiers to sap their morale and bolster the morale of the defenders escalatory? I’m not qualified to determine whether the tactical or psychological benefit is justified by the escalatory risk of these sorts of actions and in the Kerch example, we don’t even know if it was done by the Ukrainian government, provocateurs, or sympathizers acting independently.
I agree that it’s not a binary choice between appeasement and escalation, and I am very curious about the non-appeasing de-escalation strategies you allude to. That’s what we should be brainstorming and what you should lead with in your letter for it to be convincing.
This is basically a false equivalence “there are good/bad people on both sides” type of argument.
If some other country sent troops inside Russia’s borders and held a referendum for whether or not the regions they occupied want to be annexed, I would consider Russia to be the victim no matter how screwed up its internal politics are. Furthermore, such a referendum would not be legitimate no matter how honestly executed it is because the presence of foreign troops and displacement of civilians already hopelessly biases the outcome.
For the same reason, until there are no more Russian soldiers inside of Ukraine’s pre-2014 borders, I see no reason to treat these referenda and complicated stories about some Ukrainians someplace being Nazis as anything other than Russian propaganda, albeit you deserve praise for well crafted propaganda delivered in a civil manner.