I love
Isobel and this spelling especially since it complements our Scottish surname. I'd say if I were to have another dd, I'd definitely consider this although in the end, it's probably way too common (factor in other spellings) to use. I was thinking of
Mary Isobel and using both names or
Mary Bel/
Maribel. I dunno; I just love both of those names together.
Re: your combos, I think
Isobel Lucy has a better flow than
IsoBEL MaBEL.