I red those books and to be honest the IW did it mainly to themselves at least the warsmiths (re-introduced in angel exterminatus) did, I would not overestimate the contribution of Uriel and his posse.
Another example is the BL fight in the first ADB NL books, there also an example of how chaos inflicted a defeat upon themselves,
The banishing of Angron (you forget Magnus and the candle in the eye accident), I think you pass the point that there are still former primarchs active on the chaos side (Angron, Magnus and Mortarion (small reference in Pandorax), do not know about Alpharius and Omegon), taking the fight to the IOM, the last time a loyalist primarch did this was ...Dorn and he got blown to bits.
I can't recall how many GK got killed in the banishing, but a significant amount
@ COTE: I do pressume to have the same knowledge over the entire 30k- 40k as you have, but absorbing punches would be the most appropriate description,
Keep in mind that only magnus and angron were the only ones to actively do things.
Roughly 95 grey knights were killed to banish angron. Keep in mind that they could only get 100 at such a short notice. If they would have had more things would have been different.