Despite what he superficially claims, I don't see how Kant can be read consistently if you make certain duties unqualified and absolute.
He denies that any particular acts or traits are intrinsically good, except a good will. Why wouldn't that include, for example, honesty? Is honesty intrinsically good? Is telling the truth ALWAYS good? On the one hand he says no (because nothing is), but then he tries to make honesty an absolute, unqualified duty.
He is striving to create, a priori, a platonic "form" of the good. Something that is independent of any particular instantiations. Something that has it's own, independent existence and which is absolute. But his own examples belie his claims.
He is right to criticize utilitarianism's "greatest happiness for the greatest number" as an end in itself, and this is one thing he's fighting against, but he goes too far. He tries to eliminate consequences as worthy of any consideration whatsoever--at times, anyway. On the other hand, he brings in the "humanity principle" as an inherent value and an end itself.
As you note, all his "justifications" for absolute duties fall back upon an analysis of the consequences that universalization would produce. But he is very selective in the conclusions he reaches. He acts as if this is strictly a logical analysis, seeking non-contradiction, but it's not.
He argues, for example, that if lying is permitted, then there would be no such thing as a lie, and that this creates some kind of logical contradiction.
I don't find this persuasive, but that's not my point here. I would simply note that, by his own reasoning, if truthfulness was always practiced, then there could be no such thing as "truth," either.
As a result, many philosophers read him to be saying something other than what he literally claims it to be.
I think you have to do this to get any kind of consistency out of his claims. Of course you can just reject his inconsistent claims out of hand, and say that his thoughts are simply self-refuting. But he had too great of a mind to just casually dismiss him like that.