I'm in the camp that Marion didn't return.
However, if she did, this would have required a very extreme brainwashing situation.
Predatory abusers, con artists, and narcissists can sometimes create what is known as 'the cult of two'. All the usual techniques of a cult apply but with only two people - the predator and the victim. A form of hypnotic trance state is induced in the victim using methods such as fear of consequences on the one hand and the lure of a promise on the other (future faking). At that point, the victim must be 100% disconnected from the outside world with no contact whatsoever to reality, including people and all forms of media. They must put all their faith and belief into the cult leader and have absolute compliance and have lost all form of perspective, critical analysis, cynicism, and rationale.
Were that to be the case, I can't imagine how Wonka could get Marion into such a deep trance state so quickly after spending little to no time with her in the UK and how that could be maintained whilst she took flights etc. Usually it takes a sustained period of isolation / captivity (even if voluntary) whereby the victim has had zero contact with the outside world whatsoever and developed a profound terror and paranoid suspicion of others.
Sometimes men (I have yet to hear of a woman doing it) who create these mini-cults have two or three other victims that they treat like a close knit family unit, often held in isolation in one house, with everyone doing what they're told. That dynamic can bring the newcomer into the fold more quickly, as can things such as being drugged.