I tend to agree with most of the stuff you mentioned (except that I liked Yasuna). I thought the OVA sucked completely, but it did serve its purpose of resolving the story, which was extremely necessary after that 'WTF ambiguous' ending of episode 12.
The essential bits of Ayuki's character could have easily been rolled into another character's personality (maybe Sora-sensei's), and Namiko was just plain useless and annoying. I found Yasuna's yurivision a bit silly at first, as I thought it was just an excuse to get the good girl-on-girl fun rolling quick, but I liked the way they brought it around in the end.
I think the quickest way to sum up the series would be to say that if the whole series felt like the last three or four episodes, we'd have had a serious winner. The "humor" was quite intrusive at times, and characters seem to be suffering from Alzheimer's a little too much (I'm looking at you, Asuta) during the more lighthearted moments, but I was quite impressed once it kicked into drama mode. I saw a good number of love triangles when I was growing up (read: over the last five or six years
), and was even on the losing end of one, and I believed every bit of the way the whole thing played out here. For anyone who has been fortunate enough to not get themselves mixed up in this kind of thing, I can say that the way it happens in Kashimashi is very much how it happens in reality - especially when you're dealing with indecisive people like Hazumu. This is why I watched it, and I was not disappointed in this regard. I just think some of the other stuff could have been done better. I would have especially liked more development on Asuta beyond what we got at the beach (good stuff) and 'OMG my friend is teh hawt!!1,' but apparently there's a manga around somewhere, so maybe I'll have a look at that.