I personally think that Faraday is correct about his 'Variable' theory but not entirely so.
People, and maybe only certain ones, can take Actions that are Variables in the equation. Daniel speaking to Desmond, outside of the Hatch, was possible because of either or both of them being special some way. However, it was the Action, and the present Desmond's memory of it, that dislodged him from Destiny's Time Line.
Now, in the present, Desmond has taken a series of actions based off of his conversation with Faraday outside the Swan. If that event wouldn't have happened, he and Penny would have just hung out on their boat. However, because of his memory of it happening, he went to Oxford, had a visit with Widmore, went to L.A., met with Eloise and the Losties, left, then got shot on the pier by Ben.
Eloise can't see the outcome of events surrounding Desmond because his current actions, and their outcomes, are outside of Destiny's Time Line. She may be certain about the outcome of events within the Time Line, that take place in the past or future, but may eventually end up being completely surprised that they don't happen as they 'should'. Desmond, being invisible to her 'sight', may enter into events on the Time Line and muck them about. She wouldn't have a memory of that happening because it hasn't happened yet, it'd be the first time. It's even possible, once Desmond remembered his conversation with Daniel, that Eloise can no longer see or be sure of any event at all, now that a Time Sliding person outside of the Time Line exists and is most likely going to return to the Island where physical Time Travel is possible.
So, leading up to my point, the question is, why? Why did Eloise, keep Desmond from buying the ring? He was going to dislodge himself from the Time Line and change the future then and there. She knew it was possible. That's why she was in the shop, to make sure it didn't happen. Why was it so important that he go to the Island? Why did she ensure that her son go to the Island and die for apparently no reason when she knew that things could be changed?
Could it all have been to create Desmond the Time Changer?
In 1977, Faraday lights a fire under the Losties and begins a chain of events that, although intended to stop it, causes the Incident to occur. This eventually leads to Desmond, in 2004, turning the Fail Safe Key and getting his Time Sliding/Future Seeing ability. Faraday then shows up, outside the Swan, and speaks to Desmond. The memory of this dislodges the present Desmond from the Time Line and creates an 'anything is possible' future.
Without Faraday going to the Island, and eventually dying, Desmond would never be in this position.
Could this be what Eloise wants?