At the very least, it's not presented like that. It's implied that entire time line is certain. "she knew he would leave".
Well, they do present it, it's just very subtle.
Think about it. The movie is about choices, acceptance and impermanence. With that being said, regarding her husband leaving her; he leaves her because, as she says, he couldn't understand a decision that she made. That decision was the decision to have a child that she knew would die, and without telling him until it was too late; even though, she knew before she even got pregnant.
She has agency over the decision as to whether or not to marry the scientist guy, or to have a child with him. She's not destined to do either of those things.
What is close to certain is that, if she does marry him, and they have a child: then he will leave her, and their child will die, and as a result they will suffer.
She made the decision to go through with this anyway, because in her view life is worth living, even if it's only for a moment. She made the decision to have the child with him, because she loved him and their daughter and she knew both of them from the future context.
Again, think about it. If she chooses not to marry him, or not to have a child with him, her daughter that she loves, would never exist. So she chose to go through with it. And that's
why he leaves her, because of the
choice that she made.
It juts feels like a huge plot hole.
You should probably watch it once more, they do explain some of this expositionally.
On one hand we're predicting futures with certainty (we will need you in 3k years)
Again, they're seeing their own future.
For example, what if they realize their sun will nova in 3,000 years because of an unexpected astrophysical phenomena? Or, their planet will get hit with a gamma-ray burst, something 100% out of their control. While the future may be hazy and malleable within their own lives; they would all immediately be able to realize that every one of them dies in 3,000 years since no one would be able to see past that point.
Once someone had the idea to contact humans for help, their future context may have opened up and changed.
This dilemma is analogous to the protagonists dilemma as to the death of her daughter; the only difference being she accepts that death, whereas the aliens are trying to survive.
and but they couldn't see China flipping shit
They did realize that might happen. That's why they sent so many ships -- to force humanity to form a cooperative to solve this problem... Conflict would be virtually inevitable and the only way humans could achieve the end that both species needed would be to work together.
and ultimately Costello dying.
I think it was Abbott that died? But, he sacrifices himself. If you remember Costello split.. They knew immediately what was happening, and this is why they eject Louise and Ian from the ship just in the nick of time.
There would be no reason whatsoever for the other 11 ships to fuck around outside of Montana if it was consistent.
Again, they wanted to establish a relationship with the entire human species. If they only land in Montana, given how the world was during Arrival (and frankly, how it is today), that wouldn't work.
That's why they purposefully divided the message into 12 parts, for 12 ships, with each one having only a piece of the puzzle. They wanted humans to share the parts of the message, solve the puzzle, start thinking in their language, and by consequence immediately realize they were on a peaceful mission to share both knowledge and power with no hostile intent.
I get what you're saying, but it seems a little weak. Trying to have it both ways
Try to think about it from Ian and Lousie's perspective first. Then, reinterpret the film from that view. You'll see how it makes sense.