You don't take into account one thing. Putin doesn't need any domestic cover to use any weapons possible to crush Ukraine. Ask any Russian or read Russian forum, and you find out that they believe that Russia is already at war with the West.
I could pontificate on this subject a long time, I'll try not to do that.
I'm not making predictions of Putin's next move. He has miscalculated at every level. What I was saying in March was that a NATO presence makes it easier to use those weapons (wmd's). It would validate his rationale for the invasion.
Russia for the most part follows it's treaty obligations as it applies to weapons. Using chemical weapons would be a big escalation, and it would be an admission that Russia has been violating her treaty obligations under the International Convention on Chemical Weapons and the Geneva Convention.
Russia could use chemical weapons in a false flag operation. Release a small amount of something and blame Ukraine or NATO. Then use that as a justification for an escalation. That, imo is the greater risk.
Chemical weapons are used against civilian populations, and Russia already has plenty of conventional weapons that are good for that purpose. Putin can inflict mass casualties on civilians without using chemical weapons and dealing with the additional backlash that would invoke.
I do try to pay attention to the Russian state media narrative. The demonization of Ukraine and NATO has stepped up a lot in the past month. It doesn't surprise me that it's reflected (and amplified) in Russian social media.
The underlying current is that "we want this to be over". The frustration over the RF's inability to prosecute the war is pushing some people to the "win at any cost" mindset.
At the same time, Putin is finding out that it's very hard to get people willing to go in and actually fight, and a general mobilization will not be popular.
So a month later, I still stand by my comment- a NATO force presence in Ukraine would increase the risk that Russia will escalate, not decrease it. But I also think Putin is willing to escalate either way, and that's why the narrative has shifted from "special military operation" to "Russia is being attacked".
(* sorry, this was longer than I intended...)