Oh, goodness gracious! Took me a few tries, but I finally saved all the humans on the second planet on veteran. Thinking about it... the game gives me no actual bonus for doing it on a harder difficulty. I really just ought to do it on rookie and save myself all the effort.
Also, I was on the Housemarque forums, and there's one good thing that I read. For the keepers that have one guy in green and the rest in red (may not be able to see the reds), you
have to kill the one in green first! You first get that in stage two near the end with the weird floaty squid things. Also, there are some humans that require you to have a certain multiplier to free. I think the 9th human on stage two is like that as I failed to free it once, but I didn't recall screwing up, and the next time, I had a ~10x multiplier, and he just popped out.