I sometimes find them but couldn't get any of the humans to use them, who can use them? (please don't tell me it was only the Damned as I found these srolls while playing the empire campaign)

I believe inquisitors but the damned for sure. You should be able to use those runes even if you're playing empire. The rune spells are not specific, and can be cast by anyone. I've cast a few summon treant spells as empire before without issue and I'm pretty sure thats an elf spell.