Crazy Jay wrote:1) why did the Triumvirate think the Emperor was dead, Terra fallen and create Imperium Secandus?
Lack of communication throughout the galaxy. Guilliman basically reasoned that he couldn't be sure if the Emperor was alive, because of the breakdown in communications, and the scale of the Istvaan Massacre that Lorgar had told him about. And realistically, he had no way of knowing either way. Your question could easily be reframed as "why would the Triumvirate think the Emperor would still be alive?"
Guilliman's options were "run blindly into the dark to try and reach Terra, which may or may not still be around, while my forces are still reeling from Calth, with absolutely no intelligence or logistical support" or "rally forces to me, because I know I'm still alive, and we can marshal strength together, and either come save Terra if/when we find out it's still there, or avenge the Imperium in it's absence".
Of the two, I think rallying and waiting for information was the smarter option.
2) is Roboute G always an insufferable wet blanket
In the majority of his
HH books, he's a pretty chill character. He gets good showings in Know No Fear, Betrayer, and Unremembered Empire.
3) what were the other non traitor primarchs doing during imperium secundus and what did they think about it? About not being included?
They didn't know it existed.
Dorn was already defending Terra, literally being based there.
Khan had been left to his own devices largely, Horus had hoped he would side with him, but Khan rejected that. He investigated Prospero, and then went back to Terra to regroup.
Russ was held up at Prospero and later in space, and then pretty much went straight for Horus, got his ass handed to him, then had to recover with Corax.
Corax and his legion were bloodied from Istvaan, and resigned themselves to guerrilla warfare, alongside what remained of the Iron Hands and Salamanders, whose Primarchs were absent and either MIA or KIA.
Basically, the loyalists rallied in three groups - those closest to Terra, the immediate survivors of Istvaan, or those who were too far from Terra, but found the light of the Pharos guiding them to Ultramar. None of the groups were really able to stay in contact, so largely operated independently.