As for the other humans...the depends on if you think Laazar and Naugus count.
I highly doubt Naugus was intended to be human; not with that claw, horn, tail, and everything else of his (unless he was a mutant). Laazar, however, I could see as a potential human being; though that too is questionable due to his elfin-like appearance.
Remember how much Snively looks like a human.
Like I said, it depends on if you think they count. If they're human enough for you, they count. If they're not...well there ain't no humans save Snively and Robotnik.
Though I highly doubt Hurst wouldn't introduce more Human Characters, even if they were only in a flashback thing to explain Robotnik and Snively's survival on the ship thing.
Good point. Though with Robotnik and Snively, they still retained normal human characteristics (human ears, hands, bodies, ect.). Their appearances were exaggerated in the same style as the humans seen in "Unleashed" were; a Pixar-like caricature appearance.
Naugus, on the other hand, hardly looked human at all; due to his lobster claw, large horn and ears, long tail, and sharp teeth.
The only other human I've seen in anything SatAM-related was in the "Sonic Underground" episode "Winner Fakes All", when Sonic rushes past a teenage girl walking the streets of Robotropolis and blows her dress Marilyn Monroe-style. She only appears for a brief second, but it had me curious. But since most fans don't like to mention the show (much less connect it with SatAM), I guess that technically doesn't count.
Well it doesn't count for SatAM.
I'm neutral on the whole Underground thing. I like some episodes, and even some of the songs aren't terrible (specifically if you compare any to the Super Mario Supershow Songs). However even when muting the songs, the show just doesn't hold my interest. Some episodes are really good. Others....like the whole Sea-Sponge-Pirate, and the Whale, and whatever was going on in that episode, just.....well...shouldn't be there. They were a waste of time in my opinion.
To be fair, SatAM had just as many problems as "Underground" did. The only difference is that more people grew up with SatAM, thus they have more attachment to that particular series. But for me, I grew up watching "Sonic X" and later watched both series simultaneously when my interest in the franchise returned. I saw the flaws in both shows, but I still enjoyed them for what they had to offer and their different take on the Sonic mythos.
Since "Underground" was originally planned to be part of SatAM and DIC claimed that the final show served as a prequel, I just accepted it. I mean, the Archie comics are a separate entity and were only loosely "inspired" by SatAM, so they have little in common in terms of continuity (and what Hurst had planned for the show only serves to further separate them). I guess you could call SatAM/Undergound the "DIC universe" or something.