I wonder how much of Celeborn staying has to do with keeping an eye on this.

I’m not sure he thinks he would, but perhaps he thinks that the choice might not have been before him. But it isn’t necessarily about choosing a person (I have in my head Galadriel and Celebrimbor would have been not well suited, but, of course, was unlikely to result in a gruesome death.) but picking the things that go with them, in the case of Mairen, she satisfies that thirst for knowledge and creation in ways Galadriel, for all her many gifts, can’t. Choosing her enables him to do —or so he thinks and, to a degree, it’s not untrue, so much of what he wants to do. But perhaps satisfying those urges isn’t exactly healthy either.