I don't really see how a situation like that could work as a poly-fi triad - I mean, if my husband wanted to bring a lifelong friend of his I didn't really like that much to live with us in perpetuity, I would say no. If our relationship had to end over it, then...so be it. I find it odd you describe her as both reticent about the situation and bitter towards you, and are even considering living with both of them at some point (which is what I believe is the goal?). I could see he or I splitting our time living in two households if that became our ultimate desire in this situation, but I don't think anybody is ever too thrilled with that. Or I should say that most people who post about having to time-split with multiple "co-spouses" in two different homes instead of all living in the same building has seemed to be pretty stressed about it.
I certainly think that primary and secondary aren't the best words in the world but I do understand how they are useful here. My husband can love somebody all he wants, but our agreement was that we commit X amount of our time and energy to each other, and no other partner of either of ours will have more than 3x a week with us, unless we get along so swimmingly we decide group friendship time is something that's going to be incorporated to increase that amount (OR decide to change our current dynamic, which just isn't likely unless it goes from married to not married). Doesn't mean they couldn't be considered a primary partner too, it's just if they want more than that, they just wont get it.
I wonder if you have asked how he, and she, see the time being split if you live together. Just how would you like it? Perhaps she will be totally fine with things again at some point and want to spend more time with you and it will be pleasant and wonderful for everybody. If she requires 4 nights a week to sleep with him to feel OK, and you require 4 nights a week to sleep with him to feel OK, who gives? Do you know your minimum requirements that you need to be happy with the situation?
"He seemed to be taking on the view that she is demonizing me and deeply wounded, even "sick". "
It sounds like they really need to have some seriously honest talks. That sentence leads me to believe that it's not happening. I am surprised this has been going on for so many years. I do want to also say I am REALLY sad that you say this is the first time you have actually firmly stated your needs. I guess I am lucky that I make sure to do that at the beginning of any relationship and regularly, because I feel it really empowers me, and makes me brave enough to deal with all the odd situations that can come up in poly. It sounds like all three of you probably are failing to be brave enough to say the hard things that should've really been talked about.
Don't know if a good exercise would be for all three of you to write up a little mission statement document - what you would like the relationship to look like, what you can or cannot, or are or are not, willing to handle. I am guessing at this point things are so muddled that misconceptions are flying around, and what everybody really wants and feels is too wrapped up in painful feelings to be clearly understood by the other two of you in the situation.
Happiness will never come to those who fail to appreciate what they already have.