In other words: the aliens are Wights, and Louise will become Culaina the Thunorg.
They even have similar circular written languages intended to represent a non-linear view of time.
Just because they want to ally with humanity, doesn't mean that the threat that they will face in three thousand years isn't a enemy civilization or one that they enslaved. They might perceive all of time at once, and want to help humanity, but that help comes at a cost. Who knows if they have some less mutually beneficial arrangements with other civilizations who aren't the key to their salvation?
They can percieve time non-directionally, and experience past, present and future simultaneously. However, they can only see one possible future: because they are four-dimensional. They only see events in one possible timeline. It's not that choices don't exist and the future is set in stone: other timelines are possible if different choices were made: the heptapods simply can't see those outcomes as it is beyond their perception in the fifth dimension. To illustrate, think of the dimensions in Flatland, where the two-dimensional square can freely enter and exit and percieve the one-dimensional Lineland, but in turn is himself unable to percieve the three-dimensional Spaceland. Perhaps the Heptapods have similar limitations in their perspective of time and the multiversal "hyper-time".
Not that they were supposed to to maintain the challenge to spur Louise to learn their language but they correspondingly take in a lot of English during their communications and perhaps couldn't help trying to make sense of it. How exactly does that get Costello able to understand verbal English by the end? Well, the Heptapods probably also sent their best linguists.