Well, I'm glad to read my little lady isn't the only one boldly defying the written limitations of her breed.
Then again, from what I've read, Sebrights are infrequent brooders, but no one told my girls.
I am in college majoring in biology and just getting into chicken genetics, myself! Awesome to see a fellow geek!
You are most likely not going to get any white offspring as far as I know. Leghorns are dominant white, so unless someone introduced recessive white into the breed and your birds are split to it, I don't know how white would show up at all.
In regards to whether your birds are split for single comb, birds that are not pure for rose comb usually don't have very good rose combs. However, that's not a 100% indicator, because I've heard of single combs popping up in show quality Wyandottes. So, yeah, basically you'll have to breed them to find out for sure.