I'm still giggling at "corporally challenged"
I agree with what Nakis has said. I also believe ghosts can "migrate" and do tend to attach themselves to people with a lot of energy or someone who is on the same "wavelength" they were in life.
I've had things disappear and reappear before; it is a pretty common experience for people who have those "corporally challenged" residents.
It could be the spirit of your grandfather. However, do you know anything about the history of the land your home is on? New houses can be haunted also. If a spirit was attached to the land for some reason, either a place where they met an untimely end, or enjoyed a great deal in life, etc...they may try to remain there.
I did live in one home where I experienced the "rotten eggs" odor, and I know it isn't pleasant. I agree with Nakis, it may not mean an "evil" spirit, but one who may have some "issues." However, I've got to say, I don't like the sound of when you described it entering your body. I'm a firm believer that my body is my own and nothing better mess with it unless it has my permission!!
Odd as it sounds, have you thought about talking outloud to this spirit? Sometimes just having an open, frank discussion just like you would with a living human being is just the key. Tell it you know it is there, you are (or aren't) okay with it staying there, and that it needs to leave the personal belongings of your family alone. I think I would also make the statement that entering or using your body in any way shape or form is a huge no-no and it will have to leave if anything like that ever happens again.
Just remember, it is your home. If you're okay with sharing it, that's great; just make sure some boundaries are set. If not, you do have the ability to take it back and send your resident ghost packing. Keep us updated!