RIGHT?!! I was screaming when they didn't fuse. Lapis is a close second, and to be honest I think she was Blue Diamond's Pearl (the one with all the hair and stuff).
Maybe, from the sounds of it Lapis doesn't like being controlled so closely, could this be because she was Blue Diamond's right hand? But what if Lapis rebelled and was forced into the mirror, eventually finding her way to Earth through the main rebellion?