What are we to think of the shortsightedness of the great mass of people who are content to do nothing about it, and even worse, the greed or venality of the rich and powerful who deliberately bar the way to human survival?

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

In the mid-1960s, as hard to believe as it may be now, choosing to go into academic philosophy was not an imprudent career choice. There were lots of academic jobs in philosophy then.

It is absurd for anyone to think that Soviet "Marxism" is a correct application of the thought of Karl Marx. No doubt Soviet propaganda represented it this way. But who believes Soviet propaganda? It is remarkable (but maybe not so remarkable after all, when you consider their motives) that apologists for capitalism, who would not accept Soviet propaganda on any other point, are eager to agree with it on this point.

Kant's description of most ethical duties reads more like a description of moral virtues and vices. Once we see this, we see that Kantian ethics is indeed a kind of virtue ethics, and that it does not "divide the heart from the head" (to anticipate one of your later questions) but instead recognizes the deep truth that reason and emotion are not opposites.

What is central to morality is rational self-constraint (acting from duty), in cease where there is no other incentive to do your duty except that the moral law commands it.

We usually can't know how, and we probably should not even ask, how our lives contribute to a better world.