Could probably handwave that as the act of talking simply being what comes most comfortably to the host body as opposed to him willfully trying to speak as himself.
Anyway, I'm looking at the Killer Frost stuff as content for next season. They probably want to get over the whole split personality aspect they wrote themselves into, though I could see her getting her powers back during this final showdown here as I sort of "I'll save someone and they'll think I'm dead, but I'm not!" kind of move.
Also seeing them keeping stretchy man around since they lost Kid Flash to Legends. Better for power variety that way, too. They also planted seeds for Sisco going away, even if temporarily.