Thing is: humans turning into eldar-esque psykers would strengthen Chaos
At most it can be argued that they'd strengthen the power of the Warp in general, but not Chaos. It's canon that humanity will become better and kinder as they become more psychic, essentially becoming more like the Emperor. This makes sense because those who don't get corrupted/executed/both due to greater Chaos compatibility and those that remain and are more saintly will survive to reproduce. Since the Emperor is anathema to Chaos, humanity as a whole becoming more like the Emperor would only weaken Chaos.
the Emperor ascending into a full god would make him count as Chaos too...
You're assuming three things here.
1. That all gods are Chaos Gods. This is false, as the Eldar gods clearly attest.
2. That the Emperor would ascend to godhood rather than merely reincarnate like his past lives did countless times.
3. That attaining godhood is an "ascension". Humans are better than gods, the Emperor is proof of that. Him becoming a god would be
descending into godhood.
Weren't there Men of Iron that got corrupted by Chaos?
Know one knows what made the Men of Iron rebel. Chaos is a popular theory but one that even those who theorise it admit it could very well be false. I personally don't believe that it was Chaos. Many might say that with Slaanesh's gestation Chaos would be active around this time, but this isn't actually the case. Up until the time when Slaanesh was actually and completely born, the Chaos gods hadn't really been doing anything. The most they could be noted for was that their daemons were a hazard in the Warp.
-Magnus finally gets over himself and saves humanity by powering the Golden Throne.
Impossible. Magnus' soul is gone. All that's left of him is the Daemon Primarch. The being now known as Magnus the Red is nothing but another one of Tzeentch's daemons and is thus a complete pawn of the Changer.