Hello Al. These are very interesting comments that touch on crucial issues. Here are some remarks to keep the discussion going:
I think your comments capture a tension that exists in the Letters (and, in modified versions, in some of Marx's other writings). On the one hand, we have the idea - which you mentioned in point 2 - of emancipation as involving a 'rationalization' of the world. This could involve, in general terms, humans subjecting social processes to conscious control instead of being controlled by them.
On the other hand, however, we have Marx's idea - which you mentioned in point 3 - that we should refrain from positing "in thought" the ends and features of the emancipated society. In this view, socialism could only be a historical movement. The features of the emancipated society could only be posited by history, and theoretical/philosophical discourse could only be a critical assessment of the results.
Thinking about this apparent antinomy is fascinating, and an adequate 'solution' - which would include discussing an agenda for Marxist politics - would require a lot of effort. At the risk of being sloppy, here are some thoughts:
(i) The above antinomy would have to be apprehended in a dialectical fashion. A statement born "in thought" about what the future society would look like, as soon as it is picked up by acting subjects, becomes a statement effective "in history" - which can even negate it. However, this should not allow us to think of such statement as entirely immanent in history. In fact, one needs to take a step out of history to say something "transcendental" about what ought to exist.
(ii) I agree with you that the political discourse of Marxists should not refrain from making such transcendental statements. We should not be averse to moral foundations (as you discuss in point 1) and, moreover, we should try to discuss the means and ends of a socialist society. It is not adequate, in my view, to trust this task to "goddess history" or to "the masses" (which are often defined in a historicist fashion).
(iii) I hasten to add that this should not imply an authoritarian position (the "vanguard" dominating "the masses"). The transcendental principles of socialism - and, in my opinion, of the transition to socialism - should be democratic. In fact, one can maybe argue that the idea of the 'revolution' as a historical immanence contains (despite its defenders' intentions) an authoritarian principle, or at least a potential justification for authoritarianism: "we should remove those who oppose the movement of history".
(iii) That being said, I don't think we should think of socialism as a fully transparent, rationalized society, or a path for it (see my post on the Manuscripts). Although I agree 'in principle' with the idea that humans should rationally devise ways to prevent social processes from becoming "automatic subjects" that dominate them - such as the capital relation -, I don't think a socialist society should aim to banish all abstraction, unintended dynamics, etc. For example, some form of economic planning could coexist alongside some forms of markets. In discussing such system, we should of course take the 'endogeneity of human preferences' into account, as you stressed, but also the fact that central planners may lack dispersed knowledge, leave little room for spontaneity, etc.
The point is not that our Reason or our motives are faulty, but that we need to take a step out of ourselves as voluntary subjects and gain an understanding of the involuntary results of our interactions, the constraints that present themselves at a given time and place, etc. In short, if we are not be at the mercy of 'history' (the result of all this), we are not to try to completely shape it from "our" point of view. Developing an agenda that effects a "synthesis" of these two positions is to me one of the big challenges for the left.