Kant takes a free will to be a being or substance with the power to cause a state of the world (or a whole series of such states) spontaneously or from itself.

I think Fichte did take it further than Kant by arguing that we can regard the moral law as objectively valid only by seeing it as addressed to us by another being, even though Fichte thought God could not literally be a person who could address us.

I think the term "Kantian constructivism" as an oxymoron. Kant was a constructivist about mathematics, but not about ethics.

I don't think Kant's theory looks bad to people except insofar as they have misunderstood it (for instance, as heartless and ironheaded, or as committed to an absurd metaphysical conception of freedom that violates Kant's own philosophy).

I think that both Mill and Sidgwick are great and admirable philosophers, from whom we still have a lot to learn. I would not favor a form of Kantianism (if there is such a form) that treats Mill's or Sidgwick's moral philosophy with disrespect.

The relation of the law to the self is only a helpful way of thinking about the law, that helps us better understand its validity for us.

Freedom is an unprovable but unavoidable presupposition, not an article of faith.