>That you still use the nonsense oxymoronic term
Nonsense, yes, but it's not oxymoronic. It probably just comes from someone misremembering the title of Nihon Yokai Henge Shi.
An oxymoron is when you describe something with contradictory terms, like "bigly little" or something. "Hengeyokai" would more-or-less literally translate to "mutant spirit" (or apparition or phantom or whatever you choose to translate yokai as). It doesn't make a ton of sense in Japanese but it's not an oxymoron (anymore than, say, "moronoxy" is an oxymoron. It isn't, it's just misspelled).
>They would likely be lumped under the Yokai banner...
Yeah, if you're gonna pull that then dwarves should be a kind of elf since that's what they were in the original Nordic mythology. Trolls, too. But then things change over time. You don't have to make things perfectly accurate to the original folktales, and in fact I'd argue that you shoulnd't as it's frankly a bitch to try.
In the case of hengeyokai (from D&D), they serve a useful role of representing the many, many different kinds of shape-shifting animals in Asian myths - not just Japanese, either.
>spirit folk as yokai
I don't think so. Spirit folk are specifically half-spirit, half-human as D&D defines them. It's just that "spirit folk" sounds better (and more Asian) than half-spirit.
>Nezumi and naga
The Nezumi as D&D represents them would NOT be yokai, as they're not spirits or apparitions in any way, but a distinct ratfolk race. They're basically sane skaven. Naga, meanwhile, don't come from Japanese mythology, they're from India. Saying that naga are yokai are like 40k fags trying to claim that every magic in every setting is psyker warp fuckery.