I reckon the whole things gonna end up being some sort of mad paradox. Here's my thinking:
The mother is descended from someone from the OST who has travelled back in time. The candidate is the one person assigned with the task of ensuring that timeline continues to lead up to the point where that person travels back and kicks things off again. And so the loop continues. If this loop is broken, then clearly, the original timeline will cease to exist.
The 'source'/electromagnetic property of the island is what allows this loop to happen, and therefore must be protected at all costs. As such the candidate must ensure that this is protected.
'Adam/MIB wishes to leave the Island. This is not a part of the cycle so must not be allowed to happen. Whilst his body dies, his conciousness has been freed (in the form of the black smoke), which has thrown a spanner in the works. Smokey is the same 'person' from the episode, but the source has made him this way. Now the candidate must also ensure that he cannot enact his plans, and in doing so, break the cycle and destroy the original timeline.
The FST, Widmore and Ben Linus, Eloise and Desmond are for me, as Faraday would say still 'variables'. In short - f**k knows!