Perturabo's lot are siege specialists, no? Why would he use any other legion in sieges? Ok, I understand the garrison duty bit, but still, when you have orders, you execute them.
I really don't think that Night Haunter scorned the Emperor as much as the others, but yeah, you are right in this one.
Whinny little spoiled brat...that's how you portray Mortarion in this one. He is a big boy, he should have gone over it.
If the Emperor helped him, we would have another Mortarion case. Angron's warrior pride would be sorely hurt with that one.
Magnus disregarded the Emperor's warning, it wouldn't have gone any other way. Not to mentioned that Magnus ed up the great plan of Big E. And, as I've said in a post above, it was Horus who betrayed Magnus.
He isn't a God, and he had warned Lorgar numerous times before Monarchia. So again, it comes down to Lorgar's stupid stubbornness.