Lahabreha possessed Thancred's body back in 2.0. The process wasn't complete and by destroying the dark crystal that he used to do it we managed to kick him out. Elidibus has Zenos's body dug up and takes possession of it for the rest of 4.0. Well until Zenos in the body of a regular Garlean foot solider makes it back to Garlemald and the throne room. If you want to travel to other shards or to the source you have to well die. Or if you've technically already done that you need to shed any body you were in in order to do so. It's why the WoL/Wod being in the 1st whole sale is unique. Emet even tells you he did such when he went to the first and yes he took over the body of Solus. No we don't know what happens to the soul of the person if it happens while they're still alive. Most instances that we know of seem to be possession after the person died. Even Ardbert and friends did the whole body ditch when they came to the source on false pretenses of doing Elidibus's deeds thinking it'd save the 1st. Even Unhikalhi you know the strange kid Urianger introduced us who lives in the Solar of the Rising Stones is just like how the Scions are while on the first a soul manifested without a body.