Why did you even tell her there was a conflict? Seriously, this is a need-to-know basis. She did/does not need this information until her little heiney is in the car on the way to the airport. Then what could she do? Refuse to leave? Ok, maybe that could be a problem.
You're right - I should have kept my mouth shut! It was one of those "tal before thinking" things.
To clarify one point - the incident with the hotel/taxi took place several years ago, I no longer have a baby (the baby in that story is almost 6, and my "baby" is 2!). And she did take the taxi.
In general, I do think that visitors are important; when dh's elderly grandparents (whom we all adore) came to visit us a few months ago, I didn't let the kids go on playdates, and I rescheduled an activity of my own. We don't see them often, and it was a special occasion.
MIL is very big on appearances. She wants to have this scenario of happy, squealing children rushing across the airport into her outstretched arms. Even if the only people who will see this are total strangers. I'm letting reality intrude on her fantasy, and it bothers her.
I agree that dh should deal with her...and I expect he will because she will go crying to him that I hurt her feelings.

His usual response is to agree with me privately, but still want to humor her.