Theory: Good or Bad?

Perhaps possible if you can you should have some test or experiments donne.

Also I am very sorry about the situation your in.

Possession rarely occurs however when it does happen its normally the malevolent beings or the lower entities and not always demonic. Some one brought up the point that its not always demons that possess and that's very true. Usually its the a lower entity trying to make a name for itself and cries its a demon. When researching through Demonology and angelogy it normally goes back to a pagan deity broken down into layers of the original being. As far as the possession itself a person has to be willing or so weak minded that they're easily tricked into letting it go on. Its the very same principal as someone following a cult and the idea of brain washing for the lack of a better term. Ultimately free will is the only idea that can't be tampered with. Meaning it has to be given freely and consciously. There's a lot to the mental illness that often gets associated with spirits and what not. Although there's a few theories on it such why people get multiple personalities. Normally people start having issues internally and don't always deal it so it becomes a thought form ( personal shadow) and takes on the form as an entity. Those can drive a person to madness just as quickly as any annoyed entity. So really research what you're dealing with including energy signatures so you know how to deal with it before it gets out of hand.