Writing has many different possible ways to compartmentalize. Some examples I've seen used in gaming;
Single writer who finds one or more editors
Single writer who confers with a team to see if everything actually makes sense.
Lead writer who knows the skeleton of the story, then grabs writers to do each chapter/segment
Lead writer who writes the story but has other writers write dialogue by assigning one (or more!) per character
Lead writer who does the main story, but has others do side quests, optional party dialogue, etc.
Multiple lead writers that bounce ideas off each other
Multiple lead writers that then do the above things further out like tree branches
Disparate writers hired by the developer* to do different sections of the game. Usually done when the game is known how it mechanically works but the actual plot is unknown
Disparate writers adding their own vignettes to a game, nearly compartmentalizing whole sections of gameplay. (It can be done in a way that the player doesn't notice, but it's rare)
Lead writer who hires a developer to make the game their story is about but defers to others to do the more "gamey" dialogue
If you're the single writer of a game, then you need to understand that it's a lot of work. Even the best writers choose not to carry 60 hour rpgs on their own shoulders. The second bullet point is extremely important, you need others to bounce ideas on, to see if your story makes sense, to see if the structure of interactive media actually works and doesn't undermine everything.
Plus, ignore "gaming". All the issues of writing, all the quirks, the slog, blocks, burnout, and inspiration, everything applies from writing a book to writing a movie, game, television show, etc. A game is noteworthy for length and it's extremely important to not bite off more than you can chew, to be able to see that you bit off more than you can chew, and to pull back from doing so.
I mean, people checking for consistency is fine I suppose, what gets confusing is when many write different parts, and the above is indeed confusing... I bet they spend hours writing about who is going to be writing what in the first place, time which I certainly didn't have to waste for my amateur game... and the way I see it is that, even mainstream games take years to be developed, years which even books can be written in... I really don't see a problem, personally, with writing that much over a longer period. It's not like 60 hours need to be made within 60 days or something, usually... I've already written quite a bit for mine in a few months, and I needed to do everything else too, so if it was just writing I would have written even more...
no, they don't.
because if you know what you're doing, your summaries are all that is needed and those are written within minutes or one hour max for bigger areas.
a lot of your assumptions are simply wrong because you don't know how to organize data - that is the basic for speeding up any interaction after all.
and as said above: until you take the time to learn how that is done, your assumptions on how much time what task takes or how to combine work parts to a coherent whole will simply be wrong.
the same as your assumption that there would be differences to writing for a video game or writing for something else (there are, but much less than you imagine)
I have a friend who works as a writer in a game company. In one project, she wrote e.g. one character's backstory, some item descriptions and so forth. Based on what I've understood, she cooperates a lot with not only the other writers but the whole game design team: for instance, item descriptions might have space/character restrictions, so they have to be checked by programmers. That kind of teamwork also assures that the end result is cohesive.
It makes sense to me. There's a lot of "invisible" text that needs to be written while making a game, I reckon, and it's easier if people can focus on writing one thing at a time and submit that bit to used by programmers, artists etc.