It wouldn't be strange at all for Shinnosuke to be something other than human. This is Kamen Rider, and if Machine Chaser is supposed to evoke Shadow Moon (which I think I read somewhere?), I wouldn't be surprised to see him have something in common with Shinnosuke. And his tech is inspired by Proto-Drive's. (And I'm pretty sure the "shin" in Mashin Chaser is the same kanji as the Shinnosuke "shin.)
And yeah, I agree. At the very least, few people can become Drive-- the amount of effort that Mr. Belt put into getting Shinnosuke to be Drive wouldn't make any sense otherwise. Kiriko would've been a much better choice otherwise.

I just can't figure out how Tomari could've become something other than human in the timeline we have so far. And he does experience Slowdown, which I don't think Roimudes do. Certainly not Slowdowns they cause, at least.