you got that quite wrong, the reason for the precursor-forerunner war was because the precursors saw that pre humanity was more suited to the mantle, and the forerunners did not like this thus killing all the precursers and taking the mantle from them before they could give it to humanity. also, the precursors did not make the flood, they are the flood, they manifested themselves into dust as a fail-safe so they would no go extinct, but it didnt go as they planned and it became corrupt, thus the start of the flood, so they used it as an instrument to get revenge on the forerunners for trying to kill them all
the humans weren't trying to regain planets, they were cleansing them so the flood, didnt have anything to 'feed off' they had to destroy the planets instead of telling the forerunners, because the forerunners would of just evacuated, thus spreading the flood further.
im sorry, but where do you find your info, because its all wrong, have you not found a single terminal from halo 3/4 because they explain it all