Who was the ultimate originator of the conspiracy to steal the painting in Trance?

I feel the movie makes it clear that Elizabeth is the progenitor of the heists target painting, and through her manipulation of Simon, coerced him into approaching Franck with the inside information required to pull off the heist.

Elizabeth hypnotizes Simon to 'Steal Something' for her, something of great value. Knowing that Simon is an art dealer, that he has access to the Goya that is loaded with significance for the couple (and that he is potentially psychotic and has latent violent attributes) she perhaps didn't doubt that he possessed the aptitude to attempt a theft: perhaps the Goya was the most obvious target.

I don't think it necessarily mattered whether she believed Simon would pull of the heist or not, the aim was to place him in some kind of situation that would remove him from her life. If Simon failed, he would be imprisoned or killed. If he succeeded he would be living in the criminal world, or on the run. Any scenario creates distance between him and her.

Remember! Elizabeth doesn't want to take possession of the painting, she only wants Simon out of her life forever.

This is why she is so surprised when he shows up in her office. Her actions have resulted in a very specific scenario, perhaps the only possible scenario, that led Simon directly back to her office. The turn of events isn't about destiny, its about the subconscious: the mind wants what the mind wants.

Outside of universe, the Goya is only sought for its monetary value: the fact it is so meaningful to the former couple is a nice narrative addition, but is ultimately inconsequential other than as another control mechanism for Elizabeth to punish Simon.

Also, she is perhaps the best fit as to who is the movie's true 'mastermind' because she inhabits the role of that old movie trope: the unreliable narrator.

The films climax reveals that she is in possession of almost all the facts from the outset, or certainly more than anyone else.

It is only when Simon's memory returns that she learns more information, which would identify the Goya. She has played Simon through hypnosis, and the rest of the gang through persuasion and feigned ignorance.

The scene's closing moments, when she demonstrates her almost supernatural powers of hypnosis over Franck (by presenting him with the option of being brainwashed to 'forget' all the hurtful memories), compounds the fact that she is in almost total control of the situation throughout.

I didn't see this as being unclear to be honest, but perhaps I've missed some plot holes that draw discrepancies to my interpretation of the plot.

If you are unsatisfied with my answer, could you explain some of the plot holes you mention; either in your question or the comments beneath my answer?

I just watched this movie, and it was my impression at the end that Simon never really existed, and that it was Franck who had been the actual abusive ex, and that he was the one who was subsequently hypnotized not only into forgetting all of that, but also into believing that many events that had happened to him (and many that had not) had actually happened to Simon, who was in reality a completely fictionalized person suggested by Elizabeth and serving mainly as a way to further obfuscate what had really occurred as well as to create additional distance from the memory of their relationship. Hence the convenient head injury to Simon that led to memory loss, Franck being the one to suggest finding a hypnotist to fix it, Franck and Elizabeth being the ones to meet and negotiate a partnership, Franck and Elizabeth's sexual relationship being the only one portrayed, Elizabeth's insistence to Simon in the elevator that Franck remain alive, Simon ultimately dying and Franck surviving, the movie ending with Franck, etc. As far as I'm concerned, the clues are all there. What do other people think?

Personally I think the film is deliberately made to be ambiguous at many points: some scenes are presented as reality at first then one is led to discover it's part of Simon's imagination guided by Elizabeth. At times Elizabeth seems to be interested in Franck and at others to be manipulating him. Sometimes the scene focuses on Franck in a way that could be thought to be in his imaginary hypnotic fantasy. For all these reasons I think the film watcher is meant to be kept guessing, wondering who is the powerful manipulator of everyone else, and whose trance it all is. There's no shame in feeling unsure about it having watched the film, and no glory in claiming to have seen the plot clearly without ambiguity. I believe a main aim of the film is to leave the watcher with a sense of mystery and uncertainty.

She had the painting the whole time. The rest of the plot are false memories. In the last scene with Simon he sees bring it to me on his cell and the car pulls up.

I think that at the time the 4 people sat on the chair they did never wake up until the end of the movie and everything we saw was just an illusion of what they saw because when Frank thought that he was in the ocean he was actually in the pool indicating that nothing of what happened was real and that Elizabeth was actually helping Simon to get out of trouble.

Or she just made everyone go to sleep into their own fantasies and took the painting for herself.

These are the two theories I find to make sense actually.