Stereotyping itself is a problem. People complain when it exists, that it is not flattering, that it is reductive and simplistic. Yet they also complain that not making educated guesses about “types” of people betrays insensitivity. The tricky balance is that using categories is really efficient and expedient - but only if they are shared, to an extent. If I agree that PeopleX have a different culture and experience, but can’t agree upon how they do, this also upsets people. Even if one listens, empathizes, and sincerely appreciates the similarities and differences - there will still be some upset if one does not draw sufficiently general conclusions from this.
The notion that some traits are positive or negative is also a problem in itself. Some people and/or cultures appear to be eager to judge. So anything that is noticed is perceived to naturally convey a value judgement. This is where caricature becomes difficult. When does picking up on my most obvious traits amount to reducing me to those traits? Does a recognizable visual or vocal shorthand need to convey a judgement at all about the traits represented? Obviously these practices have been used to harm groups, but I think that like troubling words and epithets, it comes down to intention more than anything else.
So, relating to your third question - I think it depends. I think that there is an obligation to avoid reducing groups of people to caricature just to make a point, or ease of storytelling. Because it affects people, and it’s lazy. But it can be useful on a more “meta” level, of criticizing the practice of stereotyping. In real life, it takes a bit of knowledge and effort to attempt a sensitive portrayal of a race, ethnicity, nationality, etc. But it is still not very creative to simply put forth some fantasy people as “Space Jews” or “Dragon Rastas” or whatever. Since people are pattern-matching organisms though, I think that people will often still see parallels when they aren’t intended. This then requires some theory of mind to know how your fictional people may be interpreted as stereotypes and either avoid or subvert that process.
YES it is possible to depict the truly non-human, for most practical purposes. This has been a problem for me over four decades of reading science fiction. The problem is that what most people seem to want from characters and plots in literature is to be relatable. So you give them some visual or cultural quirks, but their motivations and responses need to be at least about 75% human (another stereotype) or readers violently lose interest. Not unlike with experimental music and film, they like it to inject enough novelty to keep things fresh and interesting, to play with their expectations rather than disregard them entirely.
So this is why we recycle the same tropes from human cultures in genre literature. People can handle only so much originality. And everything else is going to remind somebody of already existing people. As a xenophile (spell check asks if I mean xenophobe instead…) I would very much welcome more stories which made an effort to invent new cultures and organisms.