Magnus was definitely supposed to stick around afterwards. We know he was made to be able to sit on the Golden Throne both so the emperor wouldn't be stuck on it and so Malcador would never need too. The rest are pretty on point tho I think Perturabo was probably supposed to make it to reclaim more of Humanities lost technology.
Correction, the second and eleventh primarch did make it to the start of the Great Crusade as their existence was mentionned for the Rangdan Xenocides and other sources. Corax was the nineteenth after all and he asked about his brothers. At the latest they were around in 960M30 though they were probably hunted down while the records were scrapped.
I think the trouble with figuring out who was "Supposed to" survive beyond the Great Crusade is muddied because you have to figure out which primarchs had a natural inborn ability that was going to be useful after the conquests, like those suggestions that Magnus was always meant to be seated on the Golden Throne eventually (being the most psychically powerful one), or Perturabo knowing how technology worked simply by looking at it. Vs the ones who's upbringing gave them a personality or skill set that made them useful long term, like Guilliman being so good at administrative tasks, or Dorn being well suited for the days the Imperium needed to hold on to what it had, rather than conquer.
I very much disagree the emperor spoke with him for decades before he arrived on Prospero. But at no point was he told anywhere near the scope of what Corvus Corax, or the Lion were entrusted with. Magnus wasn’t even a perpetual, which means that he would eventually die. In my opinion Magnus is number 7 or 6. He was given a LOT more love than Morty, Perty, and Lorgar
Curze and Erebus are honestly tied for who i dislike the most. Though Curze is definitely a sad case at the same time. (That last conversation definitely got me a tad) But overall, i agree with yah. He definitely didn't plan on keeping them all.
I think Big E wanted all of them to survive, but repeated setbacks kept forcing him to cut his loses over and over again. I think ideally having all of them active would’ve made the Imperium an unstoppable force. Unfortunately set back after set back kept forcing Big E to either let the Primarch be lost, or set even us onto motion in order to hopefully safe where he could. If Khorne is gonna take someone, at least Big E is gonna have a say in who, and he’s gonna try his best to at least minimize the damage they can deal, if possible. I genuinely think Horus wasn’t meant to fall, given his charismatic power, he was able to bring out the best of his brothers to best suit his own goals. If anything I think Big E put Horus in that position hoping he could bring everyone together, and even reject Chaos when/if it tried anything.
I’m of the opinion that Horus was chosen since he was able to “see through” chaos if only briefly. Sanguinius or Dorn would have atomized the emperor since they’re so absolute. Horus was very “human” in the aspect of diplomacy. Which a lot of others lack. I agree on a lot of this though :)
If I may present a scenario to you what if the Mule a antagonist from the Foundation Series who is a very potent telepathic warlord led his techno barbarian kingdom against Palpatine's Empire (EU version) after Revenge of the Sith but having prior knowledge and preparations of the galaxy's state at the end of the Clone Wars and sensing public perception of Palpatine prepared accordingly using his powers to manipulate that perception against Sidious setting up uprisings and having whistle blowers appear on mass of his conspiracies and atrocities. Could the Mule conquer the Star Wars galaxy?
