You are correct, yes, they are mutually exclusive paths, Dylan at the party and Dylan/Sophia date. I didn't express myself properly. I meant that in both alternatives Ellie as LI could be introduced with similar (although mutually exclusive) spying scenes. I wrote that they "mirror" each other because you would have basically same premise in diferent alternatives/timelines. I also wrote "the scene of the previous PD", but I didn't mean that it happens previously in the same path, just that it happened in PD13 and this update is PD14. Sorry for the confusion.
So with this theory you could have Ellie involved in both alternatives:
Option A) Dylan goes to the party, has sexy time with Christine, Ellie watches, "Oh my god my brother is so hot" ->Dylan/Ellie path (but no Sophia)
OR
Option B) Dylan dates Sophia instead, the night after they have sexy time again, Ellie watches, "Oh my god my brother doing my mother is so hot" -> Sophia/Dylan/Ellie path.