Yes. Time was changed in DoFP, so now Jean's alive again. This story is set after Apocalypse, but before Logan (obviously).
As for the original question, I've heard nothing concrete, but rumours indicate there will be aliens, but not necessarily the Shi'ar as we know them. Jessica Chastain's rumoured to play the lead alien role. However, she's also said to be a shapeshifter. The name of the role hasn't been revealed. It's also been said the script is more in-line with Claremont/Byrne's original story.
I suspect Daniel Cudmore and Lamar Johnson might play aliens, as well. Although Cudmore has played Colossus, they strangely haven't said that's who he's playing for this. Maybe one of the Imperial Guard?
The Hellfire Club figured prominently in the Dark Phoenix saga. Have there been any rumors that we will see them again? Can anyone remind me what happened to Shaw and Frost after their last appearance?