[Wrana]

First, what is wrong with your approach is that you think that there is "pure" Shinto belief. Shinto IS sincretic in its very nature, just like religion of antique Rome was. And as religious activity of Chinese common folk still is. It had always accepted gods, saints and spirits of foreign origin in its cult. So yes, multiarmed deities ARE of Buddhist origins. BUT they are still worshipped by Shintoist and depicted in Shinto temples (considering "shrines" - I used "temple" here as catch-all category, but am aware that large temples are mostly Buddhist. Still, Shintoist worship in them, too. I can't say for sure whether Shintoist ceremonies take place in them, but given a nature of Far East religious sincretism, this wouldn't surprise me.)
My information is based on both literature (Ovchinnikov, Abaeve, partly Torchinov) and my contacts with some orientalists in both Moscow and St-Petersburg Universities. You can doubt me on the basis that they are mainly Sinoists, and I'm more interested in China, too, but nothing I know indicates otherwise than what I said. And your mistake seems to be not in some obscure Japan specifics, but rather in basic premise.
To HoneyBadger: mainly yes. It's not quite correct to use "good" and "evil" in respect to this mythology, as it doesn't use such terms itself. Such creatures act to restore a harmony when the balance is tilted and there is a threat to universal order. But this order isn't "peace and love" and a threat to it may come not only from the direction commonly percepted as "evil". And Buddhist religion also includes Mara, who is deity of deception and relatively close to some perceptions of Christian Satan. I don't remember offhand whether any servants of him were mentioned, but such wouldn't surprise me. And of course, term "demons" is used in western translations due to the fact that Jesuits used it first after seing depictions of such creatures...