Re: Coraline isn't made up
"Resembling coral" would be more likely, surely? Like crystalline = resembling crystal? But you're probably on to something with your Carolus idea: anyone who wanted a change from Caroline but not too great a change might well find Coraline suitable.
Whatever, I don't much enjoy adjectives as names - Bonnie, etc - so for actual use I'd far rather use or see Coralie.