My main issue is not the understanding of the text in question, it really is a question about his conception in general:
Kant defends the idea of God by hypothising a "supreme (original) good", including both moral and sensual satisfaction (in "heaven"). It seems at various points as if he would both state that respect for the moral law is the only motivation needed for moral behaviour (so that it is a suficcient condition) and this moral motivation would suffer if there was no highest good and therefore sensual satisfaction thought with it. I think this to be contradictional in some sense, but seems to rest in his very concept of rational beings.
One reference linking obligation by reason directly (and nessessarily) with the ideas of God and a future life from the Critique of Pure Reason, A 810|B839:
I term the idea of an intelligence in which the morally most perfect will, united with supreme blessedness, is the cause of all happiness in the world, so far as happiness stands in strict relation to morality (as the worthiness of being happy), the ideal of the supreme Good. It is only, then, in the ideal of the supreme original good, that pure reason can find the ground of the practically necessary connection of both elements of the highest derivative good, and accordingly of an intelligible, that is, moral world. Now since we are necessitated by reason to conceive ourselves as belonging to such a world, while the senses present to us nothing but a world of phenomena, we must assume the former as a consequence of our conduct in the world of sense (since the world of sense gives us no hint of it), and therefore as future in relation to us. Thus God and a future life are two hypotheses which, according to the principles of pure reason, are inseparable from the obligation which this reason imposes upon us.
The problem is nicely put in this article, too.
To put it another way: In the Critique of Practical Reason Kant writes that respect for the moral law is the only moral motivation (V:71-89), but later writes, that this respect for the moral law has its aim ("Absicht") in the supreme good as the highest end (V:132). Therefore, while the principle and the motivation of moral acts is the moral law itself, the end of them (as all acting must rely on ends) is the supreme good. But I think what happens then is that the moral law itself becomes the maxim, because if it was respect for the moral law and accordance of a maxim to it that suffices for moral acting, this maxim would by itself carry an end and the supreme good was no longer needed because there already is a additional motivation at hand that accounts for being finite (though it cannot be the actual subjective ground for acting in the will, that is in the rationalization of the act). Adding the supreme good is therefore some kind of overdoing it.
In Summary: Kant would not need the supreme good, but he actually conceptualizes it as part of finite rationality.
Question 1: Correct?
But then, what argumental function could it still carry so that Kant thinks he needs it beside dogmatism?
Question 2: Would it be correct to say that the supreme good helps to strenghthen what he calls "character" (Gesinnung, Denkungsart), but cannot be thought as motivational factor for moral acts?
Question 3: (more "common reason") Wouldn't it be correct to say that much more respect is earned if one seems to act on principles without needing a religious motivation? Or in a more harsh way: Isn't religious belief only needed for those who need to have a small rest of selfishness in every action to be motivated?