[S7] Who do you think control's the Riverland's?

So House Frey is basically extinct and the only people who seem to have any power is Ed Sheeran and his band of merry Lannister soldiers.

However Edmure Tully is still alive along with Roslin and his son. Is he under Lannister control in a dungeon somewhere? Could he be free and able to take back the Riverlands?