Guillimsn came back because the ultramarine are the flagship chapter, it's as simple as that and anyone thinking any other primarch would be coming back first is kidding themselves.
Well yes.
Calling Guilliman a traitor because because he (mistakenly but not unreasonably) assumed the worst and built the imperium secundus when he was magicslly cut off from the imperium and unable to do little else is pushing it. What was he supposed to do, fly his ships into the hellmouth and hope that maybe they'd make it out?
Considering that his actions prevented not just him but Sanguinius and Johnson from even really attempting to make it to Terra, yes.
He voided the Edict of Nikaea without the authority to do so; that being treason by the Emperor's own command (although others did this one as well).
He founded his own empire independent of the Imperium; this is again treason.
He violated the Codex Astartes to a greater extent than literally anyone else in history; this is by his own decree treason.
He usurped the High Lords of Terra and the Imperium in direct violation of his own decree and without any legal authority to do so; that is again treason.
So yes, Guilliman is flatly a traitor. That he had "reasons" doesn't negate his treason.
And doing so was the right call, given that when a more unified, central and cohesive structure has taken root in the imperium it's gone wrong literally every single time (the age of apostasy being the most obvious example). Yes, there's no standardized training for the IG. There's not supposed to be, because all it takes is one guy slipping a few lines into the standard training manual and now a massive chunk of the army is tainted by chaos.
Doing so was massively the wrong call. If Vulkan hadn't popped up at the eleventh hour, the Imperium would have fallen to the Beast because of Guilliman's "reforms".
As for a unified command structure going bad; well when it can only be created by individuals of extraordinary power deciding to ignore the rules to build one; I would posit that you are conflating cause and effect.
And again, he broke the legions because he feared that his loyal brothers would do what he did; declare themselves independent and build new empires. Decreeing that if a Primarch fell their legion would be divided into Chapters might have made sense, but ordering the breakup and complete removal of Astartes from all command structures was the height of idiocy. Especially seeing as he decided to totally ignore that little detail for his own empire and legion.
He doesn't have the power to do that. Even the emperor didn't have the power to do that. The AdMech is a sovereign power that is tied to but not under the imperium itself.
He absolutely had the power to do that post Heresy. The Mechanicum was in blatant violation of the Treaty of Olympus and he had six other Primarchs by his side along with nine legions against an enemy that was self destructing. What he should have done was sent the full might of the Imperium in Sol against Mars, purge the traitors, and then install Vulkan as the new Fabricator General with orders to reform the cog boys; at the time he had the force, authority, and prestige to make it stick. Instead he decided that embracing heretek traitors was a much better idea.
Actually, it's my understanding that post HH Imperial government was mostly functional, but decayed in the years since for various reasons.
It collapsed into worthlessness by the War of the Beast fifteen hundred years post Heresy. Literally three lifetimes for an upper class Imperial. It then had to be purged entirely by the Grand Master of Assassins and an Imperial Fist who flatly committed a coup; said assassin spending the next century using his agents to go on one of the largest (if not the largest) corruption purges in Imperial history.
Guilliman didn't create a government that could last or survive the rigors of time in the post Heresy galaxy. Hell, the Inquisition is said to have repeatedly in the millennia since purged the entirety of the High Lords on multiple occasions and are the ones doing most of the work of actually keeping the Imperium in tact.
I, uh, don't recall him doing that. Ever.
That's what the Primaris are. He ordered them built, ordered that only he could authorize their deployment, and that he was their commander. The entire Primaris project was multiple kinds of treason. But then, again, Guilliman has been pro-treason for a very long time now.
