My theory about Greville !!SPOILERS!!
!!!SPOILERS BELOW!!!
Greville is either the devil, or some kind of evil spirit, or demonic influence. Here's why I think this:
1. At the parties in the beginning of the movie, he comes across a man of influence that seems to know everyone's dirty secrets, despite the fact that he doesn't seem to have an official title or position anywhere. This suggests that he has some kind of powerful, secret insight into people's lives.
2. All the important and influential people he greets at these parties seem intimidated by him, even frightened of him. Perhaps because they have cut a deal with him in the past?
3. He seems to be constantly undermining people in these parties, mentioning their failures or criticisms to them in a kind of politely mocking way. Is this a way of discouraging these people from continuing with their careers?
4. All the stories he relates to Mary are either negatively charged or horrifying secrets of the great people at the center of British society. He also seems to be an insider amidst the groups to which these people belong, suggesting that he seems to be drawn or even behind this negative energy.
5. He keeps these young, attractive, bright young girls close to him in some kind of harem, perpetually at his command, almost like their will has been taken from them. Perhaps this is what he intended for Mary, which is what the key represented - essentially a symbol of her relinquished will.
6. When Mary rejects him, he destroys her career in retribution. Not necessarily a sign of the supernatural, but not very nice either.
7. He doesn't seem capable of adapting to the new freedoms of the '60s. In earlier times, he seemed to feed on those in high society who feared having their appearance or acceptance within their social circle destroyed. The freewheeling '60s and its looser morals seem to have rendered him less effective, thus his pleading to Mary for help.
8. Most obviously, he shows up several years later and hasn't aged. He also seems to have kept his harem of women, even into their elderly years. He also still seems to be attempting to undermine people, as exampled by his comments about Mary and her old age.
Anyway, those are a few of the conclusions I reached with this story.