To my knowledge, it's not just
Annabel with an M, it actually IS
Annabel with an M. Back in the merry Middle Ages someone in Scotland wrote
Amabel (good Latin name, meaning Lovable) and someone else misread the M as NN.
Short A in front is the pronunciation I know - like
Annabel, as you said. The only other attempt I've ever encountered was a sad effort to make it sound like aMAYbel, presumably because the name
Mabel is in there ...
Corollary:
Annabel looks as if it means Beautiful
Anne, but it originally meant Lovable. Trivia, but hey, trivia's fun!