Kant, the actualist principle, and the fate of the only possible proof

Commentators have repeatedly claimed that Kant offers a tacit restatement of his pre-critical "only possible proof" in the Critique of Pure Reason, downgrading its conclusion from the (objectively) necessary existence of God to the (subjectively) necessary presupposition of the idea of God, but none has successfully accounted for where and why exactly Kant thinks the proof fails as an objective demonstration. I suggest that (a) the proof fails because it mistreats the actualist principle, "every possibility must be grounded in actuality," as an ontological principle applying to the real possibilities of things in general, whereas in its critically legitimate version this principle expresses an epistemological condition of our cognition of the real possibility of empirical objects; (b) this metaphysical error in the proof occurs due to a transcendental illusion.