I don't know about Shedim, but a Freespirit would not be able to posess the shell - it would need to defeat the shaman in astral combat before it could posess him, and it can't do that because the shell is just a pile of meat on the couch while the shaman's astral form is gone walkabout.
Since you've had the shaman get lost on a metaplane, the body loses no essence, and so wouldn't die (as opposed to if the shaman had got trapped by wards on the astral plane, the body would die after <essence> hours).
If the shaman was trying an astral quest on his own (it's always better to go with a group) then he might have rigged his body up to some kind of automated healing housebot (or a docwagon platinum stabilization unit deluxe if he has the money). The reason he'd do so is that if he were killed in the metaplane he'd be disrupted back to his body which would be dying, unconscious and able to do anything about it. Lying in a doc unit, he could program it to heal him every time his condition got worse than a moderate wound, or a serious wound, or whatever. Maybe he had the
gremlins flaw. Maybe he went off on his quest and everything was fine until he took his first wound; then the doc unit healed him, malfunctioned and healed him again, malfunctioned and healed him again, etc. Each heal performed on a questing character by a non-questing character increases the quest rating by 1, which increases the duration of the quest accordingly. (Some astral quests last days proportional to quest rating, rather than hours as is usually the case.) So that bit is workable, at least.
As for whether you can cast mana spells on an inert body, the way I play it is no. Sure, the body is still alive, but only in the same way that a cactus or a mushroom is. It isn't currently inhabited by a mind so there is no mind for mindprobe to probe. See
this thread for more of that.
I think it would be a challenging character to play. You'd be permanently dual natured so you'd face the same problems ghouls and shifters do. And if you were to astrally project or go to another metaplane, you might worry that someone else had nicked off with your body while you were out.
Would you try and doppleganger the original body's owner's house, car, credsticks, wife, job, reputation, contacts? The cover of being amnesiac would help there unless they took you to an awakened doctor who wanted to assence you to see if your aura was damaged... unless you had masking?
We really have no idea. Certainly if parts of the wetware are damaged, memory formation is disrupted, memory retrieval is disrupted or both. Sometimes it is routine memory (like riding a bike or playing piano) or events memory (childhood experiences) or facial recognition or shortterm memory (did I read that newspaper five minutes ago?) But whether memory itself is encoded physically in the neuroglia of the brain, or whether it's some property of the soul or whatever, is likely to continue to be debated for just as long as it has been debated in the past.