Why are reversible reactions spontaneous?
A negative $\Delta G^0$ implying that a reaction will proceed spontaneously or a positive $\Delta G^0$ implying that a reaction will not proceed spontaneously is just a "rule of thumb" to give you a feel for what to expect.  But all a negative $\Delta G^0$ really means is that the equilibrium constant for the reaction is large (and formation of the products is highly favored), and a positive $\Delta G^0$ means that the equilibrium constant for the reaction is small (and formation of the products is not highly favored).  However, if you start out with only reactants present initially, irrespective of the sign of $\Delta G^0$, the reaction will proceed spontaneously.  However, if $\Delta G^0$ is negative, at final equilibrium, the extent of the reaction will be greater.