I agree that it may have been axed or rushed. It gave that feeling at the end. Maybe they wanted it to climax in the demon sex dungeon so much that they felt it wasn't important enough?
It's been a while since I read, but the slightly tan buff guy with dark hair, was the one who I feel would have wanted to look for the original one the most. He seemed to truly love her.
I feel that the other guys fell for the current Sera.

Where's the original Sera?
Like why they didn't look for her especially those three? They've known her since they were kids and doesn't even bother themselves to look for her. She even revealed herself she's not the original yet they still flock to her.
Anyway I really enjoy reading this one. I think the ending was rushed but the whole story turned out great :)