First, I want to say that I think JR was right on the money with Desmond, more or less. The producers have obviously pounded into our heads that Desmond is uniquely special, etc, etc. However, I think the term failsafe is not correct. I love the analogy but I think Desmond is a loophole. Also, I think it's too early to tell ultimately for who- MIB or Jacob.
Lets look at the facts. A major loophole for MIB was to get John off the island, be killed, and assume his identity. But how was he able to do that? The major catalyst for that was having Alpert talk to time traveling Locke and tell him that he will have to leave the island and he will have to die to get everyone back. We learn that future Flocke knew that Locke would be there at that moment and so he took Alpert to him and told him he had to tell Locke he would die. However, WHH right? So, you have time traveling Locke at some point in time (which is interesting that Flocke knew that Locke would time travel into his time right at that moment?!) but anwyay future FLocke couldn't change WHH right? No, if you think about it at that point in time, time traveling Locke was experiencing his NOW. MIB knew that he could impact Lockes NOW by using himself and Richard as influence from the future (unbenounced to Locke). However, it was also MIB's NOW. Or, to put it another way- bot! h Locke and Flcoke's NOW moments came together in that scene. So, because of the time traveling Locke, MIB could change WHH or at least ensure it to be so. Well Locke becomes time traveling Locke because he listens to Christian. But Ben turns the FDW. However, I think MIB/Christian always wanted Ben to turn it first- that way Locke would be on the island flipping through time and create the situation where his NOW and FLockes Now came together. John is now more convinced than ever, so he turns the FDW. Well why did they have to turn the FDW in the first place? Because Keamy's team came (which I think are against MIB- even if they don't know what that means). How did Keamy's team come? Because Charlie flipped the switch allowing the phones to work? Why did he flip the switch... because Desmond told him he was going to die. However, Desmond had to protect Charlie just long enough to ensure that it happened (eventhough he might not have realized it). That all happened because! Desmond turned the failsafe-because John told him the button ! was a jo ke. John was lead to believe it was a joke because of Ecko's Yemi/MIB encounter and the leading to the Pearl. MIB needed John to question the button so that Desmond could use the key. Creating the whole chain of events.
So, can you definitively say that Desmond is Jacobs loophole/failsafe? Could he be MIBs- or a means to an end for MIB?
I think ultimately Desmond will be used as a hero, whatever that means in Lost, but I think you cannot assume his abilities or influence has been the result of Jacob. Perhaps off the island it was, but on the island I think it all points to MIB. In fact, all encounters/influence on the island have been MIB in my opinion. I also think all encounters/influence have been Jacob off the island. These are part of the rules. However, MIB was able to enfluence the death of Locke off the island (part of his loophole) and Jacob can now talk to Hurley because he brought someone that could talk to dead people (part of his loophole).