Hi rattled64, good to hear from you again.
Getting to your last question first:
I am very interested in hearing from anyone else that has taken this path of putting it all on the table in the hopes of finding a better path forward with a relationship with a pwBPD. How do you get from rock-bottom back to a livable normalcy? How long did it take? What helped?
I have heard of maybe one (or if there are more, then single digit) relationship here where "putting it all on the table" led to the relationship staying together and improving somewhat. The key part of that choice working was the non partner's commitment to working on himself. That is to say -- just laying it all out there, with the expectation that someone you suspect has BPD will take it all in and change, isn't a recipe for success. Taking the "hail Mary" approach requires that you do boot camp too. Otherwise, the dynamic of her being the "emotional leader" of the relationship continues (i.e., once again the relationship depends on what she's doing or feeling or thinking), and that hasn't worked out very well so far.
You're here because you suspect your W has the serious mental illness of BPD. Mental illness isn't something that is improved or cured via explanations, sharing information, "getting her to see the light", confrontation, or intervention. I get where you're coming from of wanting her to engage in verifiably effective treatment for BPD -- I think at some level, it's what all of us on the boards here would want for the pwBPD in our lives. It may take a profound mindset/outlook shift on
your part, though, for the "laying it on the table" approach to have a chance. If you're up for it, this is a great place to start that boot camp

A couple of situations you described stand out to me as great places to try a new approach, so let's talk through what you shared and find areas for you to "boot camp" some tools and skills, to give your relationship the best possible chance at improvement:
She has brought it up multiple times. She really seems genuinely baffled at why I think she has BPD even after I have explained it multiple times. I keep reminding her that with or without a diagnosis it helps explain our interactions when things go South or over the deepend or whatever euphemism you like. She keeps saying "why do you not have compassion for me" and I keep explaining it is because she attacks me emotionally in brutal ways when she feels like her needs are not being met, which undermines my ability to support her and meet her needs.
Explanations don't improve BPD relationships or heal serious mental illnesses.
What does have a chance at improving connection is
emotional validation, especially stopping invalidating (whether intentional or not).
When she said "why don't you have compassion for me", she wasn't asking for a literal answer! She was communicating how she felt in that moment.
Missing the cue that she was communicating a feeling, and proceeding to "explain", was invalidation, which stops positive connection.
Finding the feelings behind her words, and validating the feelings (not any "facts" or "accusations" or "interpretations") is the #1 skill I can think of that is 100% under your control to improve your relationship chances.
Next time she expresses something like "why don't you have compassion for me", take a moment, think about how it would feel if you felt like your spouse didn't care about you, and empathize with that:
"Wow, it would feel so painful to feel like I didn't have compassion for you".
Notice that you aren't agreeing with her "facts" that you aren't compassionate. Validation isn't about agreeing, placating, lying, soothing, being positive, or smoothing thing over. It's just
connecting with how it would feel to feel that way, regardless of the "reason".
This coming week, I wonder if you can find
one moment where you normally would respond with "explanation", and instead you can try emotional validation. It's a skill -- it takes practice and trial and error. You won't magically be successful all the time, but it is so worth it.
Up for it?
This part in particular seems like a weird instance of anosognosia in a way. She really does not seem to realize how viciously she goes on the attack, and how out of left field it often is for me because she has been collecting "slights" and peevs for weeks by the time she blows up. It is like she is holding on to every injustice to her that has ever happened through childhoold and our 25 year plus marriage and all the weight and angst of every one mushrooms all at once. She is stuck on the stigma aspect and feeling like it changes the power balance in the relationship, everything that is wrong with it is her fault because of BPD. No matter how much I assure her that I know I am broken in many ways, this is where she ends up. This is a danger I knew going into it. I am not sure how/if we will ever move on from it. So I just keep trying to tell her that understanding the dynamic in our relationship and what she is experiencing when we go "into the vortex" helps me manage my own behavior, and that I am not trying to change hers.
All of that seems to me like a feature of BPD. It's not a "bug" that she doesn't realize a lot of things. That's BPD -- not some weird thing in the way of working on the BPD.
I'm seeing another opportunity that's 100% under your control for changing the pattern.
What if next time she "gets dark" and says something like "so everything that's wrong is just my fault, huh", instead of trying to assure/reassure her, you noticed the feelings behind her words, thought about how it would feel to feel that way, and used emotional validation to connect with her?
Imagine being in a situation where you felt like you were being blamed, and like others thought everything was your fault. How would that feel? Isolating, lonely, overwhelming... just plain bad or awful?
Reassurances can be inadvertent invalidation. In a "normal" relationship, reassurances can create connection. But this is BPD -- if normal relational stuff worked, this site wouldn't be here.
I wonder how it'd go, next time she expresses "feeling totally blamed", you responded with genuine emotional validation: "That would suck to feel so blamed"?
If she has read the DSM criteria, I do not know how it cannot resonate with her and how she feels all the time. Feelings of emptiness. Fear of abandonement (...which can spontaneously turn inot accusals of me being "distant" at any time), her complete lack of any sense of self and the emotional dysregulation...it is all s crystal clear to me. But when I tell her that knowing all this will help me from being reactive when she lashes out at me, she seems to be completely baffeled. "Isn't how you are describing how someone that loves another person should act anyway?". It seems to point at her being completely unaware of her behavior.
That is once again a feature, not a bug, of BPD.
Part of the mindset/outlook shift that may help you is instead of being shocked, baffled, or perplexed at how her mind doesn't operate "normally", accept that you wish to improve a relationship with a person with serious mental illness. Normal won't do it here, and constantly expecting normal is a choice under your control -- it is possible to pivot away from "but why doesn't she think like me, it's obvious to me" (which sets up repeated painful experiences of you banging your head against a brick wall) and towards "Wow, yes, given that I believe she has a serious mental illness (BPD), I'm not surprised any more that she doesn't think like me... how can I be skillful and effective in new ways of communication."
I think I read that you have a therapist -- I'd be really interested to hear what your T suggests about "mindset/outlook shift" for you.
The down side of all this is that she is now completely miserable and blaming her misery entirely on me. I am really hoping we can at some point dissect our awful interactions and get to a good place. I keep telling her that I am not going anywhere and that I am focused on my own behavior. To that end, I found a great resource for friends and family members of those with BPD called Rankin Behavioral Health. They have an 8 week telehealth based course on DBT that helps family members to manage their own behavior to improve quality of life with a BPD partner or family member. I am seriously thinking of joining it.
Externalizing blame is a feature of BPD.
She struggles to manage normal range emotions and has impaired perceptions. Unfortunately, it's not highly likely that "going back and dissecting the past" will improve your relationship.
It's OK to want a relationship where that can happen -- it just likely is not
this relationship. Only you can decide if you are OK with that limitation -- because it is a real emotional limitation on her part, even if it isn't a visible limitation like using a wheelchair.
Also --

and

about taking responsibility for improving your side of things, by taking that course! That's a great example of the "work on yourself" / "do your own boot camp" mentality. Sometimes you'll read here that "nothing changes without changes", and because you're here because you suspect your W has BPD, then you are the one who needs to be the emotional leader in the relationship. Great job taking the lead and modeling working on yourself. Keep us posted on how the course goes.
So when I think of all this it makes me hopeful, but at the same time, my own resilience is flagging after 2 straight months of emotional hell. I am tired of being miserable in the relationship. I am an optimist and tend to look at the positive side of things, but it is very hard right now. I still feel like there is no room for me or my needs in this relationship. I keep feeling like life is to short to be miserable all the time. And big picture, we have so much to be thankful for. I have so much I am grateful for. But if I want to be happy, apparently I will be alone in that. I need some sort of other outlets to build relationships. I also feel like I need to put some emotional energy into my relationships with my children, but she uses up every ounce of emptional energy I have every minute I am around her.
It
is hard

I'm not even married to a pwBPD (my H's kids' mom has many traits) and it's hard to cope with her!
In a way, it isn't up to your W if there is room in the relationship for your needs (I'm not saying this is 100% true, just offering a perspective on it -- take what fits and leave the rest). You are able to make choices about if, how, and how much you engage with her when she's dysregulated. Can you unhitch your emotions from hers, offer her emotional validation to connect when it's safe, and if she escalates, disengage and protect yourself with
true boundaries? Basically -- connect when it's safe, enjoy the good moments, and be OK with stepping away to protect yourself when she's dysregulating, allowing her to self soothe?
...
I know this was a lot, so I'll pause there. Hope some of that may be helpful food for thought;
kells76