i have explained so much other stuff to answer these questions in other posts. If you read my theory about the precursors being peacefull, then that explains why they were drvien off so easily. If they had controll of the mantle, why would they be prepared to go to war with the life that they must protect??
And if the foreruners had know knowledge of the flood, then they clearly never questioned the preucrsors about their origins. and having an entire population of graveminds/precursors is much different than the regular flood. One of my theorires explains how precursors are the most developed form of the flood, or the level above graveminds. They had wiped out the life in the previous galaxy as the regular flood, and slowly evovled into precurors. So the forerunners would have no knowledge about how the precursors had evovledsince they hadnt been created yet
with your concern about the primordial not being a preucrsor itself, didnt he say something to the Didact like "i am the last of those who gave breath to you" or something similar??
and my timeline has not been put in any particular order. I have developed my theories more and more as people have questioned them in specific parts. I plan on putting my precursors theories into a topic of their own since they are soo spread out.