IMHO-while your plan to do this in this manner is understandable, under the circumstances, it is in a way a betrayal of her trust and she is going to react to that too, in addition to you doing something that feels threatening to her.
I tend to think the first approach is better here, although expect a blowup anyway. At least with the first approach, it doesn't give the pwBPD a reason to feel "betrayed" or you not being up front, and could take some of the wind out of their sails, so to speak.
How much earlier do you think would be being up front?
My plan has me telling her ~6 hours before the event. That's not right before, but admittedly still a bit last minute.
I can still do it the previous day if I decide that's better, but not earlier than that unless I move it from Friday to a later date.
My view on this might be clouded. In my mind she will feel betrayed and trampled no matter what, and giving her more time also gives her more time to obsess and lose her mind over it - and to keep escalating to make comply with her wishes again. I see little to gain and potential for lots of madness.
The blowout will come for sure. I am basically just walking over her on this, no other way to see it. I hate to do it, but I don't see any alternatives either.
Does your family know about your wife's behavior and potential personality disorder?
They know some of it, enough I think. I don't think they would be entirely surprised if she turned up at the restaurant to make a scene.
If this were my BPD mother, I would say there is going to be a reaction to your plan- and what she might do is unpredictable. Is she safe with a one year old? I don't know.
This bothers me a lot: If she gets unstable enough and I feel compelled to stay home to make sure the child is ok... Then she gets her way by acting crazy enough. So that's not great, but on the other hand, to just leave her with the child if she's having a panic attack or something, or even to try to take the child from her and then leave? Doesn't sound that great either.