He was awaiting his judgement from the other primarchs and i do believe he done more than sucker punch him
.
That's how he got the upper hand on Dorn. In a straight up fight Dorn would have gutted his emo ass.

As for Nostramo, it had become like how it had been before he came to power and because of that he was starting to get rapists and murderes in his legion and so he destroyed Nostramo indeed but for the good of his legion. In his boots i would've done the same.
So he failed to fix his home world unlike so many of his brothers then rather then fix it and his chapter he destroyed it. Seems like he gave up and admitted failure to me.
Word Bearers vs Ultrasmurfs indeed as they fought bitterly during the Heresy.
I agree totally. And would add Imperial Fists V Luna Wolves. As Horis himself said it would be a fight without end.