I really doubt Valkyrie and Korg are dead tbh. I think they've specifically said they wanted Korg to return.
They were in the same ship...so unless they escaped just before Thanos boarded, they're most likely gone. But this is Marvel and I'm sure they'll find a way for them to be alive since they didn't die on screen.
Thanos said he killed half of them.
They also could have left before Thanos ambushed them. I don't think they said how much time passed between the Ragnarok and Infinity War.
If they were dead they would show that or their bodies for sure.
I don't think much time passed at all, Ragnarok ended with the ship finding the Asgardians ---