The Iron Warriors succeeded in their plan to inflict overwhelming casualties on their hated rivals but failed to destroy the Imperial Fists entirely. The reasons for this failure have long been debated by Imperial historians. One school argues that while Perturabo could have destroyed the Imperial Fists at any moment once they were arrogant enough to fall into his trap, he did not do so because he actually relished Rogal Dorn's anguish at living with the knowledge that the Iron Warriors had bested their rivals. Another argument claims that the Iron Warriors simply lacked the determination to make the sacrifices necessary to completely eliminate the Imperial Fists. However, what remained of the Imperial Fists proved to be a hardened, veteran force willing and able to comply with the demands of the Codex Astartes, just as Dorn had intended.
I can find these excerpts everywhere it seems. The most glaring one in this excerpt standing out to me is "just as Dorn had intended," being the result of loosing many of his men.
Against the advice of his brother Primarch, Roboute Guilliman of the Ultramarines Legion, that no single Legion could effectively combat the defences the Iron Warriors had erected on Sebastus IV, Dorn launched the campaign with only his single Legion. Even worse, the assault was launched without the extensive planning and preparations that was usually so characteristic of the Imperial Fists.
This strikes me as another mistake of Dorn. He's too wrapped up in his world and his view, rather than accepting options and realities. His lack of tactics could well be to blame for the fiasco of the Siege of Terra as Horus outmaneuvered the crap out of him as self appointed commander with ease.
Your last part is just off, of course everyone has a different perspective. I choose to believe in Dorns. What you want to believe is up to you, untill they write a book about it; its all up in the air.
I can see why Dorn is your chosen Primarch.