Well, that's where my "whatever the adult wants" rule comes in. I mean, if I met an adult who wanted me to call them Mr X, I'd do it. (Ok, I'd think they were odd, but whatever.) They want me to call them King Zorgon of Blog, fine, whatever, that's their name.
DS1 sometimes calls his Gran Sue, rather than Gran, because that's what I call her. Nobody seems too bothered.
I know what you mean about the nurses, though - what are the old ladies meant to be calling the nurses? First name or last? I think my twitchiness about this situation has more to do with the general attitude of nurses (often) than the names ...