1. MAD-e-line 2. MAD-e-lien 3. I don't think I know any Madeline's, though I know several Madison's. 4. I think it could work, though most people would pronounce it differently. If not, you could always spell it Madelyn.
I have actually never heard anyone pronounce Madeline as MAD-eh-line except for on these message boards. I have known several in real life and all pronounced it like Madelyn. I dislike the Madelyn spelling.