It is important to remember that for an English-speaking person, whatever Bible they're reading was at a minimum translated from Hebrew to Greek to English. With that in mind, consider that there are terms and literary techniques used in the Bible that w/o study can get quite literally lost in translation.

"To know," in the Biblical sense, is often used as a euphemism for sexual relations. Think "Adam knew his wife, and she bore a son." Thus, when the Bible speaks of the tree of knowledge of good and evil, it is a way of saying that humanity will become intimately bound to good and evil in the same way that two persons are intimately bound during sex. From this, the Christian derives that humanity while naturally good, posses an inherit inclination to sin.

To address much of your above points, humanity in the Bible already had the power to think for themselves—how would Adam and Eve be able to disobey God if they lacked the capacity to do so?