The brain can decide on all kinds of things, but none of the things the brain decides on matters unless the person puts something into action that causes CHANGE. So take an abstract construct as you mentioned, the brain can't understand that, it can only BELIEVE that. The problem with religions is that to much is that level. they believe things without any understanding. its a dead end,
what the brain is for is taking the abstract concept, believing it, and then applying a technique that allows for the realization of this concept, an experience in what is believed. Until this realization, the belief is not true, its just an idea that one thinks is worth pursuing. Again, most relegions, do not have a way to pursue their belief to get the realization of it and so these people are as ignorant as anyone else but more dangerous cause they think they know more.
So, the brain cannot solve anything, it can only identify a problem and decide on how to fix it. Take a simple example. You are on the ground, you are hungry, there are mangos high up on the tree. You have a problem, how to get the mangos!? you brain will tackle this, your eyes will look around and your brain will take in information and perhaps decide that its possible to make a ladder from branches laying around. Then your hands make the ladder, your body went up the ladder, your hand picked the mango. Your brain did nothing other then figure out a way to solve the problem, but the probelm was solved with your body and the ladder.
the same spiritual, your brain decides you are miserable. Your brain looks around and sees a guru (the ladder) you take what this ladder offers and use your bodies energy system, nadis, etc. and in your body the chakras have an experience that solves the problem of your misery.
I must say I´ve never understood the question about free will and can´t understand how it´s interesting for anyone. Only God has the answer, for us it looks like free will anyway, and we can never reach Gods perspective. Asking the question seems completely useless to me. For me it´s like proof of (especially Western) philosophy´s lack of interest of actually resolving our matters, it just wants to sit there looking on itself in the mirror.
Mattias