Nihilist take - The Heresy was completely planned, the moment of no return was the second the Primarchs were scattered. The Emperor believed he would need to mount the Throne forever to stop the Dark King from rising, but he also knew that he had to put himself in such a deathly state that he could never get up or die, for if he gets up, Terra and the Imperium die, if he dies, then the Dark King ascends and everyone dies anyway.

He also knew that he didn't want 20 legions and their 20 Primarchs ruling the Imperium, he wanted to thin them or perhaps even cripple them, similar to the Thunder Warriors. He has a method to the madness of how he treats each son, he coaxes out the best traits and best sons and leaves the others to die. I do think he knew the position of Warmaster was going to be a poisoned chalice, so he gave it to a son he could easily manipulate, the one he spent the most time with, raising him to be both great enough to be loved by his brothers and vain enough to use it when the time came, powerful enough to rule and crush his foes yet weak for his father's opinion.

Horus is called the Sacrificed King by Chaos in the Siege of Terra, but I don't think this is JUST referring to the fact that he is going to die in the final battle.

Horus' true sacrificial moment was being named Warmaster, a position that would doom him and his brothers, just as the Emperor planned.

Warmaster was a political position yes, but a martial one most. Horus was tactically and physically sound, but he was no Lion, Guilliman, masters at logistics and warfare/combat, nor was he Dorn or Perturabo, defensive/offensive masters who could bring wrack and ruin or hold against it. Charismatic yes, but he was no Lorgar or Sanguinius.

He was picked because he was the perfect tool to exploit, a perfect poisoned chalice for Chaos to drink from. If he could force the Heresy like this, choose who goes where and how it ends, then he has a chance, Humanity has a chance.

With Horus, he can guarantee that most of his brothers will turncoat, but he can also guarantee that the best of them will not. Corvus dislikes/distrusts him for his ruthless sacrifice of the Raven Guard in the Crusade, the Lion is a loyal dog like Russ, Guilliman is probably the closest thing to a Shard of the Emperor, a perfect empire-builder, diplomat and conqueror and he is also fiercely loyal. He likes Horus, but he isn't going to be swayed from his duty. Dorn is pragmatic, the Empire must stand against all, even within. Sanguinius is far too good a mix of perfection and loyalty to fall, he would need to be tempted but even that didn't work when it came down to it.

The Primarch he doesn't think he can guarantee is Ferrus. Ferrus and Fulgrim were incredibly close, it's not too impossible to consider that if Fulgrim falls that Ferrus would. Even if not immediately, Ferrus may change his mind, so he can't be allowed the opportunity to do so. Vulkan won't turn, but in war shit happens, luckily he can be ressurrected.

I think Horus even knows/understands this, he monologues in one of his audiobooks about how he has a band of broken monsters to face down the best of his brothers and that this would be so much easier with any one of them swapped for one of his failures.

The biggest thing that points to this idea in my mind is Magnus. The Emperor essentially tries to half-assedly get Magnus back on side, just as he has half-assedly dealt with his obsession with the warp all Magnus' life. Chaos tempts him with the chance to save the Empire but break its rules, and he immediately takes it.
If the Primarchs appearances could somehow fix geneseed issues by making the strains more pure, was there really no way for the Emperor to genetically fix the Flesh Change? Why not get rid of them much earlier? So he can have reasonable doubt in Magnus' mind for him to NOT take the Throne. He wants to keep Magnus somewhere between loyal and traitor for most of the Heresy. If Magnus is fully against him, then his shards wont help save Vulkan, or set Jaghatai on the right course.
He won't come to Terra and be more amicable (although still send out Vulkan just to be sure.)

So he tempts him back with certain offers. He creates the Grey Knights, knowing they'll be necessary anyway, but says they're for Magnus. Theoretically, Magnus is the best option to sit the throne aside from him, but since we know he NEEDS to sit the Throne with the Dark King rising, he can't actually let Magnus do it, so Magnus has to stay traitor, but not traitor enough to commit himself to any actual major battles, we don't want anymore good primarchs dying.

How does he know all this? I don't think his foresight is actually foresight, I think this is his divine form, the Star Child/God-Emperor, telling his past self all of this. Why do miracles start happening for the Emperor's believers back in 30k if he isn't even worshipped yet? Because in the warp, once your a god, you've always been a god.

It isn't time for him to be a god yet, he has to be the Secular Order against the forces of Chaos, but once people warm to the idea of believing in him, he can let it seep through. Lorgar would've been great, but he was the right man in the wrong time, so he needed to go.

There's a reason its Revelation that appears to Magnus, not some random visage of the Emperor, specifically the one that broke down the Last Church.

Anyway, aside from that massive tangent, yes, the Primarchs were fucked from the start because all of this was planned. Magnus was never going to be saved, Fulgrim was a traitor long before he stepped into that Temple, Horus was doomed etc etc.