Forgive me as I dont have time to read all the previous posts, but chiming into the OP:
The former problem was the people saw the 'mark of chaos undivided' as worshipping no god in particular, despite the number of frothing people on the internet that claim so.
Being undivided was simply having no allegiance to a particular god, but worshipping the aspects of each that they needed each day. Like the old greeks, they believed in the pantheon, but would worship poseidon before sailing, ares before fighting, etc.
Perturabo using a nurgle warp disease to infect a planet doesnt mean he is devoted to nurgle, more that he said to nurgle: "Hey, give me a disease, and I'll use it to bring suffering in your name" and nurgle went "seems legit" and gave it to him.
Iron within nails it with the gods are the emotion, as opposed to the physical manifestation of their power.
Perturabo despaired as his legion was being broken up.
He shows cunning by luring Dorn into the Iron Cage.
He shows rage in battle, and is fuelled by rage at the perceived slights done to his legion.
He shows excess in the grand and perfect designs of his fortresses.
He shows his melancholy side by acknowledging that all fortresses will fall eventually.
Likewise lorgar shows all these sides also.
Rage at the slight done to his legion
Cunning, by orchestrating the horus heresy, manipulating Horus and the other primarchs
Despair, causing him to go to the eye in search of the answers int eh first place.