Marnie Blythe sounds good. I don't think
Blythe sounds like a surname, especially because she'd have an actual surname after that!
Amelia and
Maria both sound good, although I agree that
Marnie and
Maria are rather similar. Other ideas:
Marnie Blythe Paloma - inspired by an above post about
Paloma!
Marnie Blythe ChristinaMarnie Blythe JoannaMarnie Blythe SusannaMarnie Blythe Rowena