Sorry to snag your tags, @sirigorn , but this brings up a very relevant point about the use of topics like linguistics in storytelling.
While I definitely see your point, I think you're giving Weir too little credit. He's known for his extensive research into the topics he includes, and, as someone who spends time in linguistic spaces among conlangers and such, I can tell he thought a lot of the linguistics through very thoroughly.
I think the overall issue you're running into with Weir's use of linguistics is summarised as: it's not the point.
First of all, the fact that Eridian can even be construed into English sentence structure. There are entirely unrelated languages on Earth that share way more than they should. Finnish and Hawaiian sound similar and have spellings so similar you can find Finnish compound words in Hawaiian. They don't mean the same thing, not even close, but the similarity is there regardless. Languages sharing elements by pure coincidence is not nearly as uncommon as it seems it would be. Language as we know it is simply only arrangeable in a finite number of ways.
Now, is it still very optimistic and fantastical to assume Eridian and English are significantly similar enough in construction for the communication we see in the book to be possible? Absolutely. But Weir had a choice to make. He could either spend a hundred pages exploring this issue, inventing them a complicated English Eridian pidgin and having to teach the readers this pidgin to an extent where they'd be able to follow along while reading, or he could get to the point.
Would 100 pages of alien pidgin make my heart soar? More than anything. Do I understand that this is a book made for average sci-fi readers who most likely are reading this for their interest in space and physics and biology rather than complicated linguistics, and that teaching all your readers what's basically a new language just to make sure they understand what the deuteragonist, who speaks constantly, is saying, knowing that some people straight up will never get it enough, effectively making the book incomprehensible to the average reader, and that that's not a real viable option if the book is meant to sell? Also yes.
The point of the book is not complicated linguistics, no matter how much I'd love that. The point of the book is Grace Rocky Save Stars. A 100 page intermission to invent alien pidgin is just not relevant enough, so he didn't include one. He clearly considered the incompatibility of Eridian and English but chose to prioritise readers being able to actually read the book over highly realistic linguistics. Again, the linguistics are just not the point of the book.
Oh and the "one Eridian language" thing. That's never specified to actually be the case. Grace simply calls the language Rocky speaks "Eridian" because it's the only language he knows from Erid and he has no further context to name it, like, based on culture or tribe or the head of government or whatever. While it's possible Weir didn't consider that Erid most definitely has other languages or at least dialects, I find it more likely that he simply omitted its mention. With the amount of specifics he does get into at some points, I find it hard to believe he didn't once have the thought "What about other Eridian languages?" I'm sure he had someone reading along as he wrote it who would've had the thought at least. It was again not the point and Grace has met one single Eridian and Rocky never brings up any other languages, so it stands to reason that Grace, monolingual and not interested in language, just never gave it much thought. But, regardless, at no point is it ever said that the language Rocky speaks is THE Eridian language so we can have as much fun theorising about other Eridian languages as we want.
Now, is this me calling Weir a super mega turbo genius who can never make any mistakes and thinks every thought ever? No, not at all. Iirc there was actually a mistake in some science something in the original final draft of the book, but someone read it before publication and reached out to Weir to tell him it was wrong. And Weir fixed it! So he can definitely make mistakes, documentedly so, but he has people around him to catch them when they happen. And, as for the linguistics, I feel confident saying that a vast majority of the fantastical linguistic coincidences in the book are due to linguistics and the insane language barrier simply not being the point.
Unfortunately you have to make concessions when you tell a story, one of those being that if you want the book to sell, you have to make sure people can actually read it, no matter how much it breaks your heart to not be able to spend pages upon pages exploring some idea you have regarding the world you've made. I know, I know, Finnegans Wake, but that's famous because it's so incomprehensible that scholars have been arguing about it for a century. The average reader is not reading Finnegans Wake for that exact reason. Weir is not James Joyce, he'd like us to be able to read PHM without a century of scholarly debates.
The film makes many similar concessions simply because of its limitations as a film. It has limited runtime and has to choose what is most essential to make the story work for the average viewer. And thus, it swiftly skips basically all the linguistics (so much so that it hurts my soul a bit how they do it but I can live with it. Begrudgingly). It sucks, but, in a world where the sun is dying and you have several other complicated areas of academia woven into everything all the time, linguistics simply is not what the average reader will care about. They'll care just enough to suspend their disbelief and then move on. And for most people, that suspension of disbelief happens very quickly for language because they simply don't know enough about it to know what's being handwaved.
So, while it breaks my heart, I have to concede the fact that, had I been Weir, I'd have had to make the same choice he did and omit the more complicated linguistics. Are there things I would've done differently to imply a more complex nature to the language barrier? For sure. But I understand that, as you said, for someone whose main interests lie in other places, he may have just gone "you know what, this is good enough, Grace isn't a linguistics nerd either, he would not know this stuff anyway" and moved on.
Oh and this isn't even touching what his editors and publishers etc. might've demanded of him and the book. Maybe he did write that 100 page linguistics intermission but the publisher demanded he cut it. Maybe he included a bit more linguistics to appease the linguists in the audience but that was forcibly cut too. We just don't know.
So while Weir is most certainly fallible, I think a lot of the linguistic concessions made can be attributed to the fact that it wasn't the point and, sadly, most people just don't care about it like we do.
