Don't negotiate or capitulate to seat poachers!
I think the idea of letting the FAs and the GA do their job is a good one. It's like when you have your lawyer represent you instead of talking to the other party.
I wonder if there's some social engineering way of working the situation? Like documenting it on video or with pictures. Would it be worth suggesting that you would like the situation documented so you can be reimbursed and the other passenger put on UA's "don't fly" list?
I'm not sure if it's follow-through-able but it might throw her off her game if she's a habitual poacher.
There's also the idea of using uncontrollable laugher and firmess. Act angry and stubborn and don't betray any weakness. If it honestly looks like she doesn't understand your quickly spoken invective then you can quickly switch gears and capitulate like the weak worm (aka kind soul) you are.
Just generating some ridiculous ideas. OP, you are too kind!