The Grinch is actually the same species as the Who's. He is a different subspecies though, explaining his greenness. The 'Grinch' kind of Who are actually quite numerous, but live quite a distance away from Whoville. Sometime after the events of his story, the 'Grinch' kind of Who's, for some reason or another, relocate to Whoville. The Who's seen in Horton Hears A Who are the result of generations of interbreeding between the two kind of Who's, which also explains the remarkably different Whoville in Horton's story and (in the film version) all the weird technology.
- And also The Cat in the Hat. In the movie, the crate is a portal to that speck, which he called "his world".
- Related to this, "Here" and "There" is the same world as the speck. It's not just a tiny planet, it's a microverse.
- You're quite right, but, well, since the Whos are very colorful people in the special and given that the Grinch is, aside from his green coloration, quite like the other Whos, I always saw him as indeed a who hermit. Also, the color of his eyes (red when he's nasty, but turning blue upon his redemption) demonstrate that in the Seussiverse, the color of a character can be altered by his behavior. Perhaps he's a Who whose nastiness gradually made him turn green, and not the contrary as the live-action movie strangely believe.
- I think that you're not entirely in the wrong, but you're going too far. Indeed the Whos have antennae that can lead people to think that they are insects, which make sense for tiny creatures living on a speck of dust. However, Cindly Lou Who is obviously wearing a NIGHTDRESS, that's why you don't see her legs. But she has some.
Could be a Headscratcher as well, actually. If it wasn't unknown whether Seuss actually knew French or not, but it strikes me that Grinch is the first syllable of French word "Grincheux", which means Grumpy.
I know in the film she seems older (closer to six) but if you think about it, it still could be a possibility. Her age in the film is never directly stated, and in all the other adaptations, she is clearly "no more than two." Perhaps Who children just grow up faster than human children.
She could also possibly be one of those children who grow up faster than normal. Even in our world, there are 10-year-old humans who look like they're about 14 or 15. I don't know of any two-year-old humans who look six or eight, but for Whos that could be another story.
She saw him several times before trying to steal Christmas, there's no way she wouldn't recognize him, especially when she saw his face for a solid minute. Instead she played along and pretended to not notice him to teach a lesson to her materialistic parents who thought Christmas was only about presents.
Why you might ask? Well, at best it was to be right about the Grinch, at worst it was to have an excuse to propagate mass Anti-Grinch propaganda. If you're The Mayor, you know that the Grinch, especially as a child, is both a) yearns for outside interaction (despite being awful cynical about it) and b) emotionally unstable. He knew the Grinch would show up. The Mayor also seemingly had that razor at the ready, knowing why exactly the Grinch stormed off and became a hermit. It was also very convenient for him to propose to Martha May while showing her the things he could give her that the Grinch, well, couldn't, right in front of the Grinch, knowing that he liked Martha. He was trying to goad the Grinch into going on a rampage, so that he can convince the townsfolk into remaining afraid of the Grinch. Whether or not the Grinch actually stealing Christmas was a part of his plan or not is up for debate, but it sure does help his goal in making the townsfolk remain afraid of the Grinch.