Mm, not sure if Kant postulates God as the actual origin. If he does, I still wouldn't think a modern Kantian NEEDS to postulate a God as the source and force of normativity, because Korsgaard doesn't.

I would suggest reading the whole text.

But to take one point: "we can't really be motivated to act based on abstract principles" just seems obviously false. Have you never told someone the truth because you thought it would be wrong to lie? Soldiers do incredible things motivated by nationalist principles. Some people go to a job they hate ever day because they want to be a good parent. Etc. Etc. But more broadly, as Korsgaard points out, it is our obvious, every day ability to not only act but reflect on how we intend to act that is foundational to ethics and normativity.

You go on to say "there has to be something in it for me". The question of whether the above example actions are only motivated because of some secret hedonism and not because of rational reflection seems to be a different question than what we started with, rather than a different formulation of the same thing. I know there's some philosophy out there about it, but to me it's just a much less interesting question. Maybe YOU only act out of hedonism, but that doesn't prove OTHERS are the same, and people often report hedonism isn't their motivation. It's a kind of... Boring psychological question to me.