The AU in which Simon arrives to Erid first is very interesting, since (at least for what I'm understanding) it is Simon getting to Erid out of nowhere after the Iron Lung events and the eridians look at Adrian who is depressed for been a Schrödinger widow (maybe they are, maybe they are not, there is no way to tell) and go "they need a distraction" and give them this traumatize bloody alien -who is carryng the traumas of his canon event- as a distraction, something to focus their energy and a companion.
While Grace and Rocky are trying their best to get to Erid as fast as they can because Grace is suffering from scurvy and Rocky is living in an eternal panic attack.
Meaning that when Grace and Rocky get to Erid they are in their most vulnerable state besides the trip itself.
Imagine the possibilities:
Adrian and Simon not been as good at verbal communication as Grace and Rocky, but relying in reading each other body language or just making models with xenonite (I know that Simon and Adrian are also very smart, but Grace and Rocky -in the book and movie- had a laptop to help them out at least in the start and Adrian and Simon do not have that technology because 1) eridians do not have computers and 2) I don't think the submarine, even if survived, had the technology required to do that besides, Simon would never want to get inside that thing again.
Simon who is more stable in his first meeting with Grace, because he had have time to reflect and heal in a safe space, and who had have long and very meaningful chats with Adrian.
Grace who is so weak and feverish that he can't really understand what is happening.
Simon who was ready to fight this other human he heard about for the little paradise he had found in Erid just to find a dying Grace.
Grace been scare of Simon because the last time he was this vulnerable and helpless around another human he ended been send to a suicide mission, and he doesn't have the force to fight back, or to even move by his own.
Simon having his first experiences taking care of another person, in helping them to get better, instead of causing harm.
Grace having to concentrate his whole strenght and mind to do radiation tests, because for what he had learn Simon and Adrian could have got exposure to radiation (I don't which them bad, but if Simon got there first, then they may not know about the radiation danger that the submarine and even Simon can be.)
Adrian who wants everything to be perfect for their mate to go back home and take care of them, but the scientists eridians, specially the scientists community wants to talk to him, because he is know a heroe and he brought an alien with him.
Adrian been tense and have a lot of pressure because the only human "experts" are they and their mate, but they want to remove the heaviest workload from Rocky so they just work double to treat this new sick human.
Grace having hallucinations provoke for the fever in which he forgets he is in another planet, or he thinks he is still in space dying alone, before Simon can get there to assure him that everything is fine.
Grace having a bad hallucination in which he thinks that Simon is some kind of Demon that has come from him (specially if Simon is mutated), because he was selfish and he didn't wanted to go to space and now he is paying his sins (I don't think that Grace is religious, but the experience of dying, extreme pain and fever can make you think a lot of things that you don't believe.)
Adrian walking on eggshells around Rocky, too scare to pushing their mate or asking for too much, so they are trying to give Rocky space and time to process, while they go to Simon to vent. Rocky perceiving this as Adrian giving him the cold shoulder and feeling that he is losing Adrian for a human.
Rocky feeling like he is a bad mate, because he is jealous of Simon and this new connection they have with Adrian, how Adrian looks to be in such ease with him. He knows he is been hypocrite because he himself have bond with Grace, but he can't avoid feeling this way.
Rocky going into a dark place and thinking that maybe Adrian wasn't waiting for him as he thought they would (Adrian would never, but Rocky mind is in a bad spot right now).
Rocky hating Simon because he wants to take Adrian AND Grace from him. He is not only taking his mate, he also wants his best friend.
Grace accepting that he is going to die and making peace with it, this time for sure, and Simon demanding him to keep fighting.
Simon trying to use Grace as a translator when he is conscious.
The pebbles asking Grace why has two arms, and doesn't have shine eyes or poiting teeth.
Simon leaving health amulets made with branches and leaves around Grace.
Simon praying to the Sacred Last Tree for Grace health.
Simon thinking:"If even a selfless man is condemned to have a bad end, then what will become of him?"
Simon realizing that the man who saved the stars it is not all powerful as he had thought. He is human, he is scare and he is dying in a bed.