That was exactly how I was feeling, Grey Kitty. :-) At first I wasn't sure whether the snapping at his Dad (nothing like that has happened in the 15 years since I met my husband) was a positive thing or more evidence that he is cracking even more... .
I see it as the facade cracking open, and real issues getting closer to the surface.
It is a necessary step for healing... .but who knows what will happen next. He could decide to repair the facade, or to work on stuff below. Allow him space to do it his way.
We are in agreement that not living under the same roof for now is a good idea. Actually, that was equally surprising to me last night. I don't know what that's going to look like in the long run as he has the use of our friend's place for another week. We talked about renting a room somewhere if we can find one cheap enough to alternate weeks, giving each of us a place to stay while the other stays in our home with our daughter
I once heard of a couple that divorced and felt that it was unfair to shuttle the kids from one house to the other--they didn't mess up the marriage, so they shouldn't pay the (logistical) cost. I believe what they did was maintain the larger family home... .and I think they got not one but two smaller places to live, for the parent who didn't have custody that week. It would be possible to do it with only one bachelor pad... .but more challenging!
The parents had to schlep their stuff back and forth or duplicate stuff, instead of the kids.
I thought it was a creative and admirable solution. I don't believe either parent was mentally ill either. That isn't your situation... .here are a few of the issues I see for you, which are worse with the cheaper version. (one apartment + house)
1. You and he would both need to trust the other with your belongings and privacy. Roadblock #1: He's prone to dysregulation... .dunno if his impulsive/raging behavior tends to destroy property, or if you are afraid of that. Roadblock #2: I'm assuming that you wouldn't do anything like damaging/taking his stuff or snooping... .despite this many pwBPD get very suspicious of their partner. His (unfounded) fears could be triggering for him.
2. You have a somewhat stressful time where you are likely to run into him while transitioning in/out of the house at the same time.
3. For the house/apartment solution, neither of you has a place that is 'safe' and 'yours'.
One other thought... .I don't know how you will be sharing custody of your daughter, whether it will be 50/50, 75/25, or what... .but it will impact your choices. (For example if you typically end up spending 6 nights a week with your daughter, and he typically spends 1, you might get away with crashing at a friend's place that one night a week for quite a while... .but he might need to rent a room/apartment.)
This has to be a triggering thing to discuss, so if you can avoid discussing it, that is a very good thing.
Whatever you start doing is setting some sort of a precedent... .and that will be important if you do end up in a legal battle over custody. Please post a topic about this on the legal board... .the senior folks there seem to be very smart, very experienced, and very practical, and will help protect your interests. If you lay the groundwork to provide the best possible environment for your daughter now, and things go bad later, it could save you and your daughter a LOT of time, energy, pain, and money... .and you may not need to do anything that would be provocative and risk your chances for reconciliation. Knowledge is power.