You can romance him with Rose in the band. I trigger his scenes before an confesses that she's crushing on him, so I'm not sure how her romance plays out otherwise, but there's no block. (I do feel bad though. Rose is so sweet)
i had rose in the band and it hurts my heart when she finds out. sebastian asks if everyones happy with this and basically everyone but rose says yes. she just awkward laughs then pauses and says maybe. even xander and collin are ok 9or are at least acting ok. i litrally did the scene with every possible character lol) even tho the like robin but poor rose is heartbroke :(