I think another reason Artist Caliborn can't be LE is that if he was, then that'd mean basically all possible different timeline Caliborns would become LEs, which can't be, cause there is only one.

I mean, not necessarily, it’d just mean he got surprised about something he already knew about and there wouldn’t be another Caliborn Timeline where he becomes LE. Hell we don’t even know if there IS another Caliborn Timeline aside from these theorized two, for all we know there’s only 1-2 Timelines where Caliborn is around. Contrary to popular belief, every single possible action ever and up to eternity doesn’t quite happen in Homestuck.