Well, in SM64, doesn't she write "Sincerly, Princess Toadstool... 'Peach'"? I figure(d) that either Toadstool is a ceremonial title or her nickname is Peach. As for Koopa/Bowser, I think he's either "Bower Koopa", "Bowser the Koopa", or King Koopa is--once again--a ceremonial title.
I remember getting confused about that issue a bit when, in M&L:BiS, it was "Bowser Castle", not "Bowser's Castle".