Alison, on point #1 - I think I understand why you don't wish to validate. Is it because this is a behavior he has gotten away with for far too long and you wish it to stop? Because it's a poor habit of his you feel you would be validating, not an authentic emotion?

I guess I'm torn on this one. My first instinct is to say something along the lines of, "thank you for sharing that perspective with me" which isn't exactly validating, but it's not invalidating either. But that would likely allow his blame-game to continue unchecked. I thought of mirroring what he says so it sounds ridiculous but I do not think that would help (and would start a fight and likely sound in-validating).

I'm a bit at a loss on this one because it's so ingrained in his behavior. I guess I'd just say to acknowledge his blame/complaining some of the time, but not always. Try to identify which ones may require some validating so he feels heard. But also other times perhaps just a "hmmm". The reason I say this is so that you don't get into the bad habit of just ignoring someone attempting (very, very poorly) to express something. His bad habit doesn't need to turn into your bad habit of learning NOT to validate.

Point #3: When someone is frustrated and struggling with an emotion it can be very hard for logic to be of assistance. Like a toddler with a tantrum. They're not actually upset that their cheerio fell on the floor, they're upset that they're tired and this small thing they wanted didn't work out and so now there is just not a full skill-set to express the frustration. Your husband doesn't have the skillset, so he goes into some fight-or-flight mode of just shutting down and/or fight.

So saying "What do you want to do about it" is helpful to someone who has coping skills. But he doesn't, and he can't work it out in the moment of emotion. Would changing the language to "what do you think WE should do about it?" be helpful? Imply that you'll help?

Or a simple choice? "I'd love to know how I can help. Would you like me to listen, or offer solutions?"
They say this is one of the primary differences in the ways men/women communicate. One learned behavior is that men offer solutions when the woman just wants someone to listen & validate. Perhaps this is what he is actually seeking?

I feel you've tried all this, but wanted to offer the outsiders thoughts.

Last thought: Remind me, were you in therapy for a while? Still? I'm curious if your therapist might offer you some feedback on your body language. Since the two of you are in a repetitive back-and-forth dynamic and you are changing your words I wonder if you can change your body language to match, and feedback on how you are subtly communicating may be helpful. Perhaps your husband is more in tuned to that than actual words.