A full Space Marine chapter shows up with their full power - battlebarges and drop pods and whatnot - and attack the Dark Elves.
Impossible to say, really. They could theoretically collapse Druuchi society by ganking Malekith, or the dark elves might pull victory out of their asses, or the Slaan might just blow up their battle barge as soon as it enters orbit. Way too many variables to say how it plays out.
Elven sorcerers and Slaan are serious threats no matter the tech level, but the former I think would fall anyway. Space Marines are familiar with fighting the psychically powerful and have Librarians, and more importantly all three types of elves have their really good sorcerers concentrated at the top, doubly so for the dark elves.
As for the Slaan, you're thinking too much like a dirty human. The only threat to have emerged that caused a direct correlation of Slaan awakening was the imminent destruction of the entire planet via warpstorm meteors. Everything else it's just a tossup, who knows what the Old Ones told those crazy frogs.
But yes, in terms of recruitment:
Humans obviously get educated on Our Glorious Emperor
Halflings and Dwarfs 99% get recorded as abhumans and are made to feel bad for the sin of being born.
Wood Elves have a chance of being called abhuman until the whole tree thing comes out.
High Elves get exterminated like the wytch assponces they are.
Dark Elf peasants might have the best chance of being abhuman of the elves, they're kind of terrorized by the magic wielders and some might crave some Emperor distant oppression over Malekith's very immediate and painful oppression.
Ogres
maybe...? They look a lot like Ogryns even if they're much, much less...stable. Might go to the effort if there's a Magos Biologos around to figure out they dissolve Chaos and Just Imagine The Possibilities!
Very low chance of going anywhere with the beastmen, no matter what 3rd edition says.
"Even in this blasted hellscape of a planet I cannot escape the damned greenskins, Emperor why ;-;"
Weeaboo dragons and Mayan lizards get super burnt, of course.