She was always the grandma. It's not like she turned into the grandma because the girl said "Grandma."
The dude who was kidnapped likely had his brain morphed with the old lady's husband. As it was told, they weren't brainwashed, they put the white person's brain in and it controlled everything but the original brain had to be left in a bit for motor functions but that person would be in the sunken area and just a passenger (unless something triggered the brain to take over.)