Reason, God, Freedom of Choice

Having undermined, in the 1st Critique, all traditional proofs of the existence of God, it is historically significant that Kant's alternative effort, in the 2nd Critique, be sound.  Now, that proof entails the proposition that Virtue is the worthiness to be happy, which, in turn, entails the thesis that humans possess a power of freedom of choice.  However, that that power includes the possibility of choosing to disobey Reason, which Kant does not dispute, seems to present a serious challenge to his ambition, i. e. because that thesis seems to be a premise that is independent of the resources of Reason.  While Kant himself seemingly remains oblivious to this potentially conclusive lacuna in his procedure, Silber, in his introduction to Religion Within the Limits of Reason Alone, in his attempt to bridge that gap, agrees that freedom of choice is irrational, proposing that "the irrational is a mode of the rational".  However, given the absence of any further elaboration, that seemingly contradictory proposition achieves the opposite of what Silber intends--it reinforces the suspicion that Kant's proof of the existence of God is as unsound as the others that he refutes.