As I commented on the series page, I believe that Adachi was something like a ghost that haunts Shinagawa (and Monshiro high), just like the story of Hana-ko of the toilet. So, to reunite with her after so long it had to be on the toilet.
I really want it to be a happy ending of sorts, but between this and the mental broke down theory of Shinagawa I prefer this one, and that can explain some of the strange things surrounding Adachi and his family and why the author leave it on that. And this way Shinagawa is not crazy and can live with Adachi around.
Man... I just completed the last chapter and was shocked with "that ending". I really believed that the author was going to be more solid on the final story.