Usually, I try to look for a unity that the three Critiques combined could form. This idea is based mostly on two things. First, Kant 's concept of reason and the idea that is the same reason and the same logical procedures that should appear in the three Critiques (even though there could be some small differences). Second, the fact that Kant introduced in the Critique of Pure Reason the Chapter titled The Canon of Pure Reason which already contains a big part of the moral theory that appears in the 2nd Critique. In the Canon the idea of duty (i.e. the idea of another type of causality) and the concept of highest good already appear. And the definition of the highest good is the same in the three Critiques. The meaning of the subtitle "On the final purpose of reason" (CPuR B edition 825 A edition 518) is almost the same as the term "primacy of pure practical reason" (CPraR AE 119-120) from the second Critique.

I focus more on the similarities between the three Critiques and less on the differences. Unless the differences are big like in the deduction of Critique Pure Reason.

A key difference between the first, the second and third Critiques is about where the progress towards the Highest Good is supposed to "take place", even though the moral law is not 100% reachable (Kant says "holiness, a perfection of which no rational being in the world of the senses is capable at any point of time in his existence" (CPraR AE 122). Should that progress take place in a world beyond this one?, in this world but in another life? (I say progress because that is the concept that Kant uses for the postulate of the inmortality of the soul in relation to the highest good, since it is a process that does not end). In the 1st and in the 2nd it seems often that Kant thinks that, for reason, the highest good belongs in another world beyond this one, (or you could say that maybe it is just not clear if he thinks of this world as the type of world where the highest good could be looked for). But in the third that changes a lot and he is no longer talking only about Newtonian laws in the world but also about finality, in this case beauty and organisms.

So, in the second Critique the progress of a person towards the highest good is thinkable but it is not that clear for the reader if Kant believes it is possible. In theory all that could be a fiction or narrative of reason that has no real outcome but in the third Critique nature is almost, for Kant, talking to human beings ("nature figuratively speaks to us" CJ AE 301). But one has to consider also that the phrase about the starry sky appears in the second Critique already and it really is about finality, but Kant maybe does not know at that moment how to explain that type of phenomena, or he is saving that for his next publication.

In the case of my first essay, one of the central ideas I focus on is Kants explanation about hypotyposis (CJ AE 351-352), which presents the explanation of the use of symbols as the the representation of ideas of reason. It is through symbols that they seem to acquire objective reality, and Kant says that reason developes an interest in that (CJ AE 300). At a first moment,  intuitive representation for the ideas of reason seems impossible until reason looks, not for examples or schemes, but for symbols. For Kant, beauty works as symbol of the good and "only in this respect does it please"(CJ Paragraph 59) and it is only through symbols that knowledge about God is possible (CJ AE 353).

About the incalculable gulf: Like I say it looks like a simple explanation that is why it will seem improbable to be new. I am not saying that other researchers have not understood what the highest good is or where the gulf comes from. What I am saying is that Kant has tried to define logically all the words he is going to use in his system, to the point that if the reader checks the definition of the highest good, he will see that it contains the concept of happiness but conditioned by morals, and the concept of happiness at the same time contains the concept of nature. So, if one follows the definitions one will see that in the highest good nature has to be compatible with morals. That is why a new nature is needed in which at least its form is compatible with the idea of ends  (CJ AE 176).

The Newtonian nature from the first Critique gives either physical satisfaction or problems with other people. Maybe Kant is already thinking about the purposive side of nature in the Critique of Pure Reason but he does not explain it any further. Morals, for Kant, tell us about ways to perform actions considering human beings as final ends and, also, considering our relation with other people in the world of the senses, that is why the whole idea of law appears later in Kant. And because reason is always the same reason, the pure concept of the highest good and the foundation of a system of law should, in theory, for Kant, be compatible with each other. Otherwise it would be as if Kant were saying one thing about reason one day, and a very different thing another day and that, in theory, can´t be because he is always supposed to be talking about the same concept of reason. (And I do not remember anybody having metioned that the foundation of law should be, from Kant´s perspective, be compatible with the idea of the highest good, I have this idea since the year 2013, but I have not checked new literature about that)

But the point is that the concept of highest good requires a new nature because human beings exist as beings that participate in the world of the senses and in the noumenal world (because they have reason)

I think I would have found this type of logical explanation through definitions of the gulf in Guyer or Forster and the other interpreters if they had explained that the same way I do. It is almost as if Kant had in his mind an own dictionary for language in general, and for his works.

Concerning the concept of subjective necessity in judgments of taste, I assume that if somebody had solved that problem of necessity in the third Critique, that would have been mentioned in Motta's book Kant's Philosophy of Necessity, and also in Guyer's work and the work of other interpreters.  Kant says that  the subjective necessity of reason looks for the conditions of the highest good, and the condition that renders the highest good possible is the objective reality of the ideas (as postulates). This necessity is subjective and very different from the objective necessity of the categories and from the objective necessity of reason (or duty) (CPraR AE 125). And it is very different from the habit David Hume talks about, which Kant says is another type of "subjective necessity arisen from frequent association" (CPuR B127 AE 105). 

The subjective necessity of reason towards the postulates and the possibility of their objective reality is a subjective and, at the same time, logical necessity, it is a logical procedure.

Kant could be consciously or unconsciously trying to keep the unity in his works, and he could be trying to adapt a new book of his to an older one. I believe that it is a good idea to follow that intention because that makes the whole project more interesting. And, more or less, you can see a general idea of what he is trying to do even if there are differences. I am going to repeat this: The title “On the final purpose of reason” from the frist Critique is almost equivalent to the term “the primacy of pure practical reason” from the second, in both passages Kant talks about the impossibility of reason to reach knowledge about the ideas in the speculative use of pure reason. But for Kant, that impossibility disappears when one considers also the use of pure practical reason, and he starts talking about that in both passages, and he begins to talk about the postulates. It is in the use of reason in relation to morals where reason can think that it has found answers to the problems about God and the immortality of the soul as postulates for the highest good and that appears in the three Critiques, not only in the second one. So, they cannot be treated as complete different works as if Kant were changing his mind every other day. I notice the differences often, and I may even write something about that. But I think it is interesting to find out what his general project is, because it is supposed to be based on reason. One can find other philosophical texts or worldviews that are not really based on reason, and they can be very interesting. But the whole point in Kant´s work is that his theory is based on reason.

I am not trying to force the readers to believe what Kant says in his books, I am just defending my argumentation because my quotes are right, and any reader can verify if those quotes exist and if they make sense, and at this point should be clear that those three books are really connected. A way to prove me wrong would be possible if I had put the wrong quotes but that is not the case.

Maybe people that do not feel comfortable with the topics of god or immortality of the soul should refrain from reading Kant´s work.