I would say that with Connor Guerrin, there's a bit of a difference. Connor might not have been 'fully' possessed, as he was a normal child for at least three and a half days before the Warden and Crew fight their way through the bowers of Redcliffe Castle and encounter Connor and his lying mother in the throne room. It's hard to say what level of possession there is considering there are several pieces of evidence.
1) The Templar's Daughter (DAII): In Year 4, Ser Thrask asks you to save his daughter, who fully turns into a twisted abomination when cornered. This is supposedly when a demon fully takes you and mutates your form into some mockery between your old self and the demon that now possesses you. To me, this is a one-way none-reversal trip.
2) Evelina, The Good Mother (DAII): In Year 4 I think?), you meet a lady mage who has been possessed and is harboring orphans and runaways in the bowers of Darktown. While clearly told that she's possessed, Evelina is human (if a little insane) when Hawke and Crew confront her.
3) Eremond (DAII): Yes, trying to find the Templar Recruit who turns into an abomination with no explanation as to how that happened, or better yet, who stuffed a demon into him (as very likely it wasn't a mage). He was fully human and not exactly chanting Charles Manson quotes when you reach him on the Wounded Coast with Ser Cullin being rather belligerent, but then turns when heavily accosted. Possessed? Yes. Fully? No... no I don't think so.
4) Sophia Dryden (Warden's Keep DLC): Probably the best example of possession with little to prove otherwise. She's a ghoulified Warden walking corpse thanks to the two centuries of being tainted while also being possessed by what is likely a demon of pride. There is absolutely nothing left of Sophia Dryden thanks to the conversation you have with her before you attempt an exorcism with swords and spells.
5) Reverants and Shadow Warriors: These are supposedly possessed non-mages. Shadow Warriors are likely possessed rogues (as they can stealth and like stabbing you in the back) and Reverents have that extremely annoying 'pull you halfway across the battlefield to impale your ass silly' ability that are obviously possessed warriors (perhaps even possessed Templars). This is a non-refundable trip.
6) Arcane Horrors: Spell-users who have been possessed to the ultimate degree, and lash out with magic in their twisted forms. To me this is the next level up from Abominations, who just beat and claw you to death. Perhaps a timeframe is needed for evolution.
7) The Wardens of the Western Approach (DA:I) This is clearly of a different vein as the Wardens were more or less turned into demons themselves instead of merely possessed, turning into rage demons. Because of the permanent tear in the Veil (the Breach) as well as Cory-Thingie's knowledge, this is more of an exception than a rule.
8) Demon summoning (DAII): Danarius, Fenris' former master, unleashes more than a few shades and horrors against you in the first act when Fenris hunts down Denarius. Now... there's no one in that Hightown Mansion, so thus it is likely they were summoned by pure blood magic and small tears in the Veil. But no possessions. Actually, I can't think of a Tevinter Mage ever being possessed (crazy? Yes. Possessed? I can't think of one).
9) Cole (DA:I) Despite being bat-shit crazy to where even Sera thought he was nuts (and that's saying something), Cole is a Spirit of Mercy possessing a body of a dead mage (who isn't rotting away). Obviously, this is irreversible unless somehow the body gets destroyed, perhaps taking Cole with it.
10: Kristoff/Justice (DA:Awakening): Congradualations, a walking corpse Warden with a spirit animating the body. Despite answering every horror movie in existence, let's have the smelly rotting guy fight on our side! Clearly a possession, Justice has completely taken over as Warden Kristoff had been dead for at least a few days when he took over.
11) Anders/Justice: Something of an in-between possession or a merging, Anders fully admits that, while he ISN'T an abomination, he's rather close to it. Though he retains his mind, personality, traits, and skills, he also possesses the abilities of Justice as well. How Anders is different from everyone else is likely because Justice doesn't 'seek' to take over permanently, probably due to their prior friendship.
12) Wynne: Wynne was, in fact, possessed by a Spirit of Mercy, and a Spirit Healer. Despite being an obvious Loyalist and asskisser to the Templars, being a half-abomination seemed to be okay with her as long as she wasn't gutting little children. She ultimately 'releases' the spirit to save the life of a Templar named Evangeline, whom you see in 'Asunder' and is spoken of by Cole in DA:I (the missions involving Wynne's son, Rhys, and the Templar Evangeline).
13) Merrill: Who in God knows what the hell Merrill was, consorting with demons, performing blood magic, and trying to fix the same mirror that ruined Tamlen and Mahariel regardless of what Origin one picks. When a Dalish Clan throws out one of the People as well as one of their precious mages? That should tell you something.
14) Fenreyal (sp?) (DAII): The elf-blooded dreamwalker. Hawke and Co enter the Fade to stop a Desire Demon who clearly SUBERTS your crew one at a time unless your approval is high enough (in which the only time I was ever successful with that was with Bethany). This SHOULD turn them into abominations or something, but everyone returns to the real world just fine.