Fully agreed. That'd be a challenging, philosophical ending. Would be kind of beautiful to be raised alongside Tsuchikare as a twin sister and their respective swords beginning to share each other's power. Yeah, you lose on the "childhood friend romance" angle if Ureha is having to help raise them but I miss endings where romance doesn't actually happen despite being built up. Lost loves is a lost art nowadays.
In theory, yes, if there was still enough time to explore it in earnest. But my point is more that Souji is so bland and the story is almost over that even if he didn't turn back it wouldn't make much of a tangible difference if we're getting a glossing over status quo ending regardless.