einstein sent this to curie in 1911 when she was being harassed by tabloids. it contains everything you’d want in such a letter:
(1) your haters are trash
(2) you’re a baller, a true queen
(3) i have determined the statistical law of motion of the diatomic molecule in planck’s radiation field 🧪⚛️
This is a personal letter, not a professional one, would be very weird to address someone you know personally to ask them if they are OK as Dr… (at least I have never done that to my colleagues known personally).
I'm a dude and I speak to one of my lady friends using the respectful plural form sometimes. Granted, she's not a doctor, but if she were, I wouldn't address her as such because it seems irrelevant between friends, somehow. You do have a point, but it seems to be a case of friendship over formality.
Which is why the "Mrs" still bothers me. Could be a German convention that friends drop the title. But to me its just respectful. One person mentioned he used Mr. for a man, so I guess that was just Einstein. So it makes it a bit better and not so much he is dismissing the title she earned.
Stan is quite right, there's no way that he would put "Marie" there if he's using the "Sie" form. They were "per Sie," period, so it was last names only.
It's very treacherous to apply 2024 US practices to 1911 Central Europe in a different language.