That, like you said, it's common. I'd use
Isadora or
Arabella much quicker than Isabella.Isa is pretty, and you hear
Bella a lot, so that's nicer. Especially if the I is pronounced like EE.I'm not sure how many
Isabella's I know, but it sure is cropping up quickly. I don't know any Isabela's.I like the form
Isabelle best. Also, the Irish and Scottish forms are pretty cool (
Isobel,
Iseabail,
Ishbel,
Isibeal, etc.)
Yeahhh, I wouldn't use it. And personally, I'd use the mn space as a place to give names that either you just need to fit into the combo somehow because you undyingly love it, or because it's too pretentious/unusual for a fn.