Big-time fan here, and I thought I'd throw my dart at the board. Even though I'm sure Dennis Kucinich has a better chance at becoming our next president than any of this being plausible.
I'm really hoping the writers aren't going w/ a time traveling theme regarding Des. I've never been a fan of it in general. However, I'm along for the entire ride, and it has been a blast.
My take on Desmond David Hume is that he developed some intense precognitive ability via means of the turning the fail safe key. I don't believe there was time traveling happening whatsoever in the episode. The episode's title "Flashes Before Your Eyes" is in regards to the events that unfolded in his subconscious as he turned the key. He was fusing island elements into the lineage of his entire life.
I believe that Ms. Hawking wasn't anything more than a figment of his subconscious...keeping him away from what he deemed as morally desirable, a life w/ Penny.
Which leads to the reference of David Hume...I cannot see how this isn't a conincidence...taken from Wikipedia on his interpretation on Free Will vs. Determinism...
"Hume's view is that human behavior, like everything else, is caused, and therefore holding people responsible for their actions should focus on rewarding them or punishing them in such a way that they will try to do what is morally desirable and will try to avoid doing what is morally reprehensible."
Desmond was punished for performing the morally reprehensible (betrayal of self), has acknowledged, and will reward himself by an attempt to right it. (All within the lenses of David Hume)
And to toss in Hume's "Bundle Theory of the Self"...
"When we start introspecting, "we are never intimately conscious of anything but a particular perception; man is a bundle or collection of different perceptions which succeed one another with an inconceivable rapidity and are in perpetual flux and movement".
It is plain, that in the course of our thinking, and in the constant revolution of our ideas, our imagination runs easily from one idea to any other that resembles it, and that this quality alone is to the fancy a sufficient bond and association."
Which again, leads back to the title of the episode.
My small take on the episode in general.