As for Descartes argument for God, I think it sounds reasonable to me. If we were able to create ourselves, we would have made ourselves flawless beings, but since we are not flawless because of the free will thinking and false beliefs, we bring into our lives.

As far as free will and false beliefs, I think that it is true that we are responsible for our false beliefs but how do we know that they are truly false? Where is the rating system, who is in charge of right or wrong beliefs? And since God is a flawless being, I could see were our misuse of free will-the difference between us and God-could cause the false beliefs.

As for doing our bests' and ending up with true and justified beliefs because God is good, I think that it would be extremely hard to break everything down to the bare bones to build a foundation of absolute truths. Life Descartes himself argues, there is things as free will and false beliefs that by their very nature are hard, if not almost impossible to do without.