Makes total sense! What I run into a lot with my partner is that it's not so much language or tone that gets him, it's the content. So no matter how lovingly and neutrally I say something, he will sift through it until he finds the accusation...which triggers him.
I can see this happening as an avoidance and control tactic (although highly unlikely it is purposeful). If conflict is terrifying to him, then chances are good he will do anything humanly possible to stop the impending argument (and this is completely subjective as there may not even BE an impending argument). So he could be sifting through to find an accusation so that he has more control than you do to blame it on you. Perhaps him being blamed for anything at one time was a completely heart and soul crushing experience and he just can't take the cues that he sees (that you don't) without slipping into 'attack first before they kill me' mode.
Speaking of cues.... I am not sure if supporters are actually aware of how freaking subtle the cues may be. For me it wasn't so much about tone or body language -- it was all about the eyes. I could see an annoyed look in someone's eyes for like a nanosecond and be sent to the moon. The other person wouldn't even know they had done it and I was already feeling like and engaging like I was needing to protect myself. For me, that meant passing out. Or running into the cold and dropping into a ditch or whatever the closest place to hide was. So you may not actually realize that you are setting off alarms in his head that he is in tune to like you would never know....
Also, have you thought about writing when you speak about stuff? Like the two of you sitting with a pad and paper and writing words down instead of verbalizing them? It is a totally different state one has to be in when one is writing. It requires cognitive thought. And I am going to say that if he is triggered, chances are good you aren't dealing with a ton of cognitive distortions so much as you are dealing with out and out fear and panic and defensive stuff. And I have to say, based on experience, there is hardly a worse feeling than feeling that one has to defend oneself when you know you are so spun out you can't. You worry about using the wrong words, saying something that will make things worse, not being able to take in perceived accusations --- seriously, there are not words.... honest.
With writing, however, you can:
1. See when he is losing it because he won't be able to write anymore. That could solve the problem you both have of not being able to see when he is simply incapable of proper cognition/verbage etc.
2. Slow down the conversation tremendously, which will help with overwhelm and allow him (and you) to pace yourselves.
3. You can write down something like 'Nobody needs to win' or something to that effect to remind him that the end game is to allow him to express himself only. Nothing else. This may allow him a safety net and also a way of you being able to see how he is doing.
4. It takes away the trigger points that he is most likely used to -- escalating voices, frustration is voices, eyes, body language. It literally changes the whole 'fight scene' that he has been conditioned to react to.
5. Allow you both to be engaged in a solution rather than an argument.
6. Best to have some 'good' conversations via notes as well so that writing doesn't become a trigger. "Wanna go for ice cream?"
Just a couple of thoughts. It is lovely to see people who care so much about their spouses. Gives me faith in humanity, you know?