I like
Isobel well enough particularly the combo
Mary Isobel, but I'd say overall, that I'm getting over the entire set of similar names:
Isabella,
Isabelle,
Isabel,
Isobel and especially Izzie /
Izzy. I do find
Belle refreshing so it would make sense to me that
Isabelle would be the spelling for the full name.
Isobel Charlotte is lovely but seems incongruent to me because
Charlotte is so French while
Isobel is so Scottish. And yes, I'd use
Charles and
Charlotte in my children's names as long as they both weren't fns.
Isobel Jessamine is lovely. It truly is.
Jessamine is so unexpected in the middle.