I think you can NOT do the harem path and just choose individual girls' paths, but I don't think the others are getting an "early" solo ending ... but that's just a guess on my part. I haven't seen anything definitive.
Based on how things happened, it seems to me Emma's solo ending was always planned as a non-cannon side story, but it almost feels like it wasn't always intended to be an in-game choice. Maybe it was once planned to be bonus content that could be unlocked? Who knows. I love how it was implemented though.
That said, the solo ending's epilogue has some spoiler material that's only been hinted at in the main storyline ... that is, main storylines. Be interesting to see if that tidbit of information turns out to be cannon or was just NiiChan having fun.