Well that was certainly a really interesting series. Reminded me quite a lot of both Madoka and Steins;Gate. (Also there was a third thing there that it reminded me of but I forgot what it was about half way through the story and haven't been able to remember it since, lol.) I do wish it elaborated a bit more on Yukari though since it seems to be implied that even in this final timeline she's actually aware of what Gaku did for her. And the bit about Gaku becoming unobservable yet Yukari still being able to sense her is something that felt like it should be elaborated on some.
Also I might be overlooking something but it does seem like there's a fairly obvious plothole with the phone: If Gaku talking to herself is merely symbolic and she actually gets the memories and knowledge and such from her alternate selves as soon as they start talking then surely that call should have given her a ton of knowledge about the situation?