Defensively the best player suited to play CF besides Mullins is McKenna who might be gone when Hays returns. Defensively Mullins gives this team a lot in CF even if he's not hitting.
Hays or Cowser can cover CF to give Mullins a rest, but there's going to be a drop off in their defense. Until the Orioles have another player capable of playing defense in CF Mullins is going to be the starter. One position player move I wouldn't rule out is Elias getting a utility outfielder who can competently play CF.
Yes, in the way you always do: Luis Hernandez is five for his last 16. We should monitor the situation closely to see if he's really going to be a .312 hitter over the rest of his career (despite no other evidence that this is in any way likely).
Probably not quite that high but if he’s going to be a big K guy, miss bats, keep the walks down and throw a ton of GBs, that’s basically the dream profile.
Now, he obviously has a ways to go and personally, I agree with Tony that he needs to go Aberdeen but the profile is what you want.