In my opinion, for a poly relationship to truly work, all the parties involved have to love each other. That wasn't the case in this manga so I never saw a poly ending as an option. If the manga went there, it would have felt forced to me.

There are plenty V's in poly relationship. Where one person has a relationship with two other people (with everyone's consent of course) I myself am in one such relationship. I have my wife, and I have my "mate". My wife 100% supports me whenever my mate can come visit, and she gives us time to ourselves. My wife and mate care deeply about each other, and even love each other, but not in "that way". I could absolutely see that be a thing with these 3 as well, as there was a lot of mutual love and respect for all parties involved, even if not all connections were romantic ones.