Pack, when BD happened for me I was about to turn 49 years old. I got married when I was 29. All through my 20s I dated lots of different girls (even with the aura of a long-term on-again, off-again relationship with a woman that jerked me around for years).
I point that out to say that perspective on things changes a lot when you're 49 vs. 29. You are still so young and there are so many other people out there, that it pains me to see you so all in on this woman that seems doggedly determine to leave you. I am sure you've heard the old saying that "we want what we can't have" (which is a truth underlying why I allowed the woman mentioned above to jerk me around for so long). And I think that truth sometimes is compounded by the fact that now there is an OM. Biologically we turn into a rutting buck when another buck starts to move in on our doe. That competitive fire comes out in us. "Oh no you dint!" attitude bubbles up and we start comparing ourselves to the "other buck" to try compete with them. I suspect you've got a little bit of that going on. It was difficult enough when you thought she was just walking away out of unhappiness, but when another buck show up your "fight" instinct kicks in.
I guess what I am trying to ask is that is the desire to fight stronger than the desire to hold fast to a boundary? If I had asked you 3 years ago "What would you do if your W cheated on you?" what would your answer be? Would you have said that you'd have walked? Would that have been a deal-breaker? If so, what is different now?
And yes, from her perspective she likely doesn't see it as cheating. Cheater never do. Cheaters do all kind of mental gymnastics to convince themselves (and by extension, others) that the marriage was already over by time the cheating took place. It also speaks to the different the LBS and WAS views separation. LBSs see separation as a necessary step towards R. WASs see separation a step towards D. You probably didn't see separation on 10/27/19 the same way she saw it. You thought "some distance will let me detach easier (it rarely does) and that will help me bust my D." She saw it as a chance to be free and do whatever she wanted (yes, that includes dating and sleeping with other men).
So Pack the question I have for you is what are you trying so save here? You are young, have 67% of your life ahead of you, and breaking the bound you still feel towards her will only make you stronger, and let you move on. Look at the others here whose sitches have ended in D, they are so much better off today than they were on BD, and the weeks/months that followed. I like the PIES approach you laid out. Great way to look at your path ahead. But my question for you above still stands. Keep PIES in sight, but what are you planning to do to move on (and forward) with YOUR life?
M(53), W(54),D(19) M-23, T-25 Bomb Drop - Dec.23, 2017 Ring and Piecing since March 2018