The clones take the place of Elio and Glordon, and they convince their parental figures that they are the originals (to varying degrees of success). Elio claims to be Earth’s leader for most of the movie
No but an alien is put into an exoskeleton that is covered in spikes on the inside. No discomfort is seen. Later a different alien exits their exoskeleton and we see they are covered in scars/holes from the spikes.
No, unless you count Elio's clone, who dies to let Elio and his aunt pass in a military base. (It's not portrayed as sad and the clone doesn't seem affected, as it's not human nor really alive)
Along with two characters vomiting (vomit not seen) during montage, an alien computer 'vomits' out a drink, an alien regurgitates their friend who is hiding in their mouth and possible other gagging moment.
The spaceship is cramped, characters also crawl through tight tunnels and at one point a character climbs inside another character's mouth. Also a character is put inside an exoskeleton that is covered in spikes on the inside and this slowly closes on to them.