...locking horns with a strong-willed opponent who is willing to test that boundary at every turn, finally winning that battle, then addressing the underlying issue.
I do think you and I are largely on the same page, and that most differences are likely just a matter of language and understandings of what's happening when.
The other piece is that I don't experience boundary-setting (any point in that process) as locking horns or winning a battle. When we first start setting boundaries, we can feel like we're being aggressive (even though we're not),
only because we've generally been passive (or P-A). We feel like we're shouting, or being mean, or stancing for battle, even while our volume, body language, and words are entirely appropriate and peaceful. It's so foreign to us to speak up, most of us have some version of an anxiety attack the first few times. And then we go through what Beattie calls after-burn: the guilt, fear, angst that comes with having set one. So, lots of emotions and sensations while, objectively, we are being perfectly appropriate.
But once we acclimatize to boundary-setting, and those internal reactions to our new functioning are no longer happening, we feel peace and wellness. There are still no horns, no locking, no battle. i.e., Boundaries rather than walls. We're centered, we speak from our calm truth, out of respect for ourselves. We're simply presenting who we are, and loving ourselves enough to feel strength of personal, quiet conviction. It's not from an angry place, it's not an attack, it's not a move of offense. It's a sharing who we are and what we will and can live with and accept in this relationship.
Yes, for me boundary-setting or laying out boundaries is a process, based in love, and incorporating strength, patience, zen, clarity, openness, and curiousity. "Shutting down" aggression can still allow for all those other things to remain present. In my experience, aggression often needs to be shut down in order for there to be enough room for everything else.
Panda has spoken multiple times with Mom. Responded to the requests as is. Done her very best to communicate. Listened to her mom's rants. Offered her mom other options. Understood that her mom wants Panda to have a different value system. Maintained the relationship regardless of struggle and pain. Has done all this over the course of years. Might there be room to understand even more? Absolutely. There always is, in anything, for any of us. But Panda has expressed the fatigue and stress she's now experiencing this far in. When a person has done all this over the course of years and is now very tired and stressed, I think she's done enough for now and is eligible (so to speak) to set a boundary around the specific aggressive activity. She can do this directly, politely, clearly, and firmly, and still be chock full of love, curiousity, and openness (not to that one behaviour but) to her mom.
Like you say, a person isn't obligated to do more (redirect, serve as therapist, do some hours of reflective listening, seek even more to understand, etc). I think Panda has tried hard for a long time, and is pretty tired from the behaviour now. That seems like a good time to move forward to "shutting down" the specific behaviour, getting a good rest, and then deciding what she wants to do from there. But for sure, a person might also continue to try other things instead, too. Totally legit, if one is up for more as you note.