Will people be able to flash out of Camille's ult?

Don't think so, it says that you cannot leave it by any means. I just saw Redmercy playing some Camille and he ulted an Ekko, who then ulted with his ult ghost outside of the field, and he stopped at the edge of the field without leaving it, so if Ekko ult doesn't get you out of it, I don't think flash will.

No, you cannot. (Barring the case that this is another of Riot's contradictory type issues.)

The only way you can get out is if you knock Camille out, or kill Camille, or die.