After some (very peaceful) time apart, she now wants to talk by phone. I'm prepared to be as emotionless as possible to avoid arguing. I have my short list of talking points and a statement to use if I need to step away. Any other advice?
Sure, keep your part of the dialogue focused on you. "I want....I need...I feel...I think..." That's less likely to spiral than if you say, "You always....you act like....you need to stop...etc".
For example, "You need to be nicer to me" or "You're always arguing over the dumbest things" could be said as "I feel like everything I do upsets you lately." The first will probably start a fight, the second will definitely start a fight, but the third isn't assigning blame and is a much safer choice.
If she becomes hostile or starts yelling, my go-to with the BPDs in my life is to say, "I care about you and I don't want to argue." I ALWAYS lead with that. 'I love you/care about you AND...." When you say, "I care about you BUT...", they somehow only focus on what comes after 'but'. I think it's because their brain is seeking doom and gloom, so that's what they find.
If they keep trying to argue, I repeat my go-to and add a little more. "I care about you and I don't want to argue, so I'm going to get off the phone for a bit. I'll call you back in 30-40 minutes." See the pattern? I...I...I...I. It also sets a boundary in a kind way.
Couples therapy can help, but you have to go into it with the right objectives and goals. If it's to win an argument, then you'll lose every time. If it's to get the therapist to take your side and tell her that she's being unreasonable, then you also lose no matter what. Because if he does tell her she's being unreasonable, then she'll likely feel like she's being ganged up on and never return.
However, if the goal is to actually work through conflict and build trust, there can be positive outcomes. Again though, you have to stop arguing and make it clear that you're not interested in that.