What would happen to a troll pretending to be a higher bloodcaste? Would they get culled or would they just be stripped of their sign?

Obviously culled. When has Alternia ever had mercy on lowbloods? Unless whatever Highblood found them out was so amused they wanted to keep them around. But whenever you ask “Would this be a cullable offense-” Yes. That’s the point. Everything is a cullable offense if someone wants it to be.