Well, having written further on the crack fic, let me add that there would be this precedent: Secret or promised conversion a la Charles II (Stuart) ("I'll make it public on my death bed, or any minute now; meanwhile, Louis, keep sending over money"). In practical terms, no one would convert, but MT would have an embarrasment weapon if Fritz ever invades her territory again - Charles had to promise in writing; mind you, Fritz could cry forgery and invade anyway, which she knows he's capable of, but still, it would be something.

Anyway, she's now done her best to troll Fritz into wanting to troll her back by having designs on her son.