i bolded the bits I want to address.
You say that every choice taken already exists...so hypothesize about whether there exists a reality for every decision...how did the universe know to create this reality and leave out all the decisions that didn't happen?
If the universe created the realities to exist then that decision couldn't have been made under free will, because it had to have happened to justify the existance of the universe that existed to accomodate it.
If the universe created all realities for every decision, then no free will can exist, because every decision is made every time and every outcome exists to accomodate it. Free will is then an illusion. If you say that the realities exist until a decision is made, then where does the energy go when the universe that isn't needed disappears?
Is time linear? I think not, indeed I think the only sane interpretation of QM is the Transactional Interpretation. Essentially every event sends information both forward and backwards in time as the equations are symetric. (Where the 2nd Law, time and entropy fit with this is the big question), however, the major consequence of this interpretation is the complete lack of free-will, and perhaps this is why it isn't very popular.
However i didn't really want my main point to be about freewill, I am more interested in the mechanics and logic of the multiverse. I dont fully appreciate why people believe it at all.
Well I think you're not understanding. We don't create these worlds when we make a decision. They already exist and represent every possible decision we could have made. Everything that can happen does happen somewhere. Every outcome. They exist in these other parallel worlds. The universe didn't have " know " anything. It's just that every action and outcome exists. We are merely one of them. And I didn't come up with this stuff. This is all part of modern theory. It's called M theory ( it used to be string theory ) but it's a modern way to explain how things work and possibly the basis for a grand unified theory of everything ( what Einstein was working on until his death but could never solve ). In M theory the idea is that all of these parallel worlds are membranes that tend to undulate and when they come into contact that's what causes the big bang. Now the just so you understand. The big bang didn't create the membrane but it does create a condition inside of it that we perceive as our universe. There might be membranes where this doesn't happen at all. Without the math it's very difficult to grasp the entire picture of how this works. But don't feel bad I don't have that kind of mathematical knowledge myself. But I love to read about the theory even with my limited understanding.
Here's a little something from the Science Channel : http://science.discovery.com/stories...-universe.html
About the free will thing. It's a matter of perspective. Inside this universe you make a choice to have a chicken sandwich for lunch today. No one made that choice for you. However if time happens say all at once and not the past, present, future way we perceive it your choice has happened/ happens/ will happen all at once. The linear part is an illusion on our part because we can't see it any other way ( any more than we can't see that matter isn't really solid it mearly appears that way to us ). There are many things in nature that we know just aren't the way we perceive them.
Anyway It may be unchangeable but inside those parameters it was still your choice. No one made that choice for you. Also in some other universe you choose tuna. In another you had soup. Every possible outcome is represented.
No one knows for sure that this is exactly the way things are but it's the best answers we have for now and it solves a lot of problems like the paradox dilemma that makes the idea of time travel so unworkable. And of course if this proves out someday then they might be asking where all these parallel worlds came from. But that's the thing about science. There's always a deeper mystery.