If the Tartaros people seal the demon inside Mira's body, will she be able to control it as she controls her other demons?
I don't know what you mean by sealing the demon, but recent chapters showed that
Mirajane is able to control any demons or demonic things at Tartaros except of the Etherious. Including the demon they wanted to turn her into or the device they used to turn her into a demon.
It doesen't really matter if she will or she will not be able to control all her inner demons, as long as there are enemies powerful enough, that might not be a problem... and for the sake of the show, she shouldn't controll all her inner demons... because she will always have enough dangerous demons inside her... not sure if there will be enough enemies for them all...
Oh, and I'm curious what kind of creatures will Lisana take over? Maybe a dragon?