Very interesting! The only thing I would take issue with at the moment, though, is that the parts from Higurashi Outbreak that you referenced are only in the anime version, I believe, and therefore of dubious canonicity.
But this makes me want to read Kamikanshi again and see how well it matches up.
Edit: Hmm, it looks like what Rika and Hanyuu said in the anime version of Outbreak fits with Kamikanshi, although not quite in the same way you were saying.
(Kamikanshi spoilers) [spoiler] According to Kamikanshi, there are many “gods” like Hanyuu all over the earth who parasitize humans, but most of them are harmless. Hanyuu’s people, however, can cause negative side-effects in mentally unstable people. So Hanyuu and her people shut themselves up in Hinamizawa until they could develop the ability to live with humans without any negative side-effects. Hanyuu doesn’t really seem like she aims to colonize the whole earth, but I could be wrong.
But in Outbreak and Kamikanshi, Takano alters the Hinamizawa virus so that it spreads like wildfire. I think this is what Hanyuu is referring to when she says she can’t control her brethren. [/spoiler]