For Eloise what about Elle as a nickname? I know you’re wanting to call her by Eloise and hoping it won’t come to nicknames, but maybe if there was a nickname you guys liked more he could get on board with Eloise
I just saw another forum where she was also looking for a sister name to Isla. It has 20 some comments including one I left so I’ll post it here because I think you might like a lot of the names people have said on hers!