1 and 2. I pronounce bother Mad-eh-lyn or Mad-uh-lyn. I remember one of my aunts calling my mother Mad-uh-lane.
3. Personally I knew two- my mother was named after her grandmother
Madeleine who was Alsatian. On that side of the family I've found at least four or five generations bearing the name
Madeleine. My father's mother was Madaline.
I've never personally met anyone with the name so to me it's not too common, but maybe every girl I hear being called
Maddie is really a
Madeleine.
4. I would think that Mad-eh-lyn for
Madeline is just fine.
To me, any spelling other that
Madeleine just doesn't look all that right to me, but that's what happens when you grew up with that as the main spelling of the name.