Will Mirajane be able to handle the demon inside her? 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.