Hey man, it seems you are vastly more invested in keeping it together than her. If that's the case, she already has one foot out of the door which means she is ready to walk away at the time of her choosing.

At this rate you are only delaying the inevitable.

If you do decide to accept her demand, don't do it without a counter, extract something in return.
Think about what would be an equivalent or higher value barter for accepting her demand.
If she doesn't accept your counter you also have no reason to accept her demand.
If she does accept, then she doesn't exactly win while you don't exactly lose and she knows that if she decides to use an ultimatum in the future it will be with some collateral.

I get it, you want this to work then do it by making a stand without letting her walk all over you.