**SPOILER WARNING FOR DEVS**
Wow, it has some distinct similiairties, even down to incorporating the "Many Worlds" theory of quantum mechanics I see! It turns out that the machine in Devs is also kinda limited to the perspective of the user - especially in the case where it can see past Lily's death - which there happens to be no explanation for and is another thing I'm kinda bugged about in terms but in terms the consistency of the sci-fi it presents. If the machine only achieves "clarity" of its visions with the Many Worlds theory that Lyndon incorporated into it (because it was all grainy and unclear when adhering to the deterministic theory that Forest wanted for it), then there should be no reason it can't see beyond Lily's death nor should it be a "shock" that Lily changes the way she ends up dying compared to that prediction/vision.
Indeed. A single computer in a single universe would be very unlikely to have the ability to calculate infinite complete universes, let alone just multiple universes.
They never do explain how they are able to view things anywhere at anytime. What exactly is doing the viewing?
The thing is, the definition of "future" depends on how you view time itself. Most depictions of time/time travel in fiction actually subscribe to the many worlds theory (but occasionally try to grab bits of determinism which makes it nonsensical) in that time is not a single continuum but many/infinite parallel universes running at different places in time (with or without other differences to the originally shown universe/timeline). Changing the "future" in this instance is not really changing the future of the continuum you originally come from but a parallel universe's/alternate timeline. In a true deterministic view (which Forest and Katie believe in but are actually wrong and for some reason choosing to be a knowing slave to it), the future is indeed unmalleable and knowing the future is incorporated into the illusion of free will such that it becomes a self-fulfilling prophecy. Not much pop fiction have done this correctly, the only one springing to mind being the original Terminator film (which T2 then later subverts).
It's to do with the reveal that all he wanted out of the Devs (Deus) Machine was to create a simulation of a world in which he could inhabit a world where his daughter Amaya was still alive. It's makes it kinda unclear why Forest is a hardline determinist in order to achieve that because it is unnecessary and counter to this goal. In a true deterministic simulation, Amaya is and would be dead still. Before we know the reveal of this simulation thing, it makes sense Forest wants it to be deterministic like that (despite Amaya being dead) since it absolves him of the guilt of being responsible for Amaya's death because that means he really had no choice (the analogy given being he was on tramlines). The Many-Worlds theory would put that guilt and shame back on him because it would imply he had a choice and signify that he made a bad one leading to Amaya's death. This is why he's upset with and fires Lyndon.
Thing is, the only way a simulation of Amaya still existing/being alive at this present/point in time depicted in the show, can only actually occur with the Many Worlds theory (since determinism will always have Amaya dying in the car crash) and he should've known that from the beginning if that was the true intent of his making the Devs machine. He rants at one point that he denies the many-worlds theory because it won't be his Amaya but this is actually what he really wants since a simulation of world where Amaya still exists is technically, and will never be, his Amaya anyway! The only justification I can come up with is that a deterministic interpretation means that his resurrection in the simulation would mean he'd be guaranteed to be in the one simulation of Amaya still being alive and not be elsewhere in some other alternative universe but that kinda falls apart since it wouldn't matter anyway because each instance of himself in a many-worlds simulation would still have a high chance of one of him, if not infinite version of him (which are all the same person up to when he died in the real world), in a simulation with Amaya still being alive.
Also, it makes no sense why he'd be so eager to see through the so-called deterministic event of having Lily die when anything past that point is unknowable/static. You'd think he'd want to avoid that and look for an alternate timeline/universe (because Lyndon's work in the Devs machine should allow that) that will see him enter the simulation he wanted.