I very much agree with human flourishing as the main value I most want AI technologies to pursue and be used to pursue.

In that framing, my key claim is that in practice no area of purely technical AI research — including "safety" and/or "alignment" research — can be adequately checked for whether it will help or hinder human flourishing, without a social model of how the resulting techologies will be used by individuals / businesses / governments / etc..

I may be missing context here, but as written / taken at face value, I strongly agree with the above comment from Richard.  I often disagree with Richard about alignment and its role in the future of AI, but this comment is an extremely dense list of things I agree with regarding rationalist epistemic culture.

That is, norms do seem feasible to figure out, but not the kind of thing that is relevant right now, unfortunately.

From the OP:

for most real-world-prevalent perspectives on AI alignment, safety, and existential safety, acausal considerations are not particularly dominant [...].  In particular, I do not think acausal normalcy provides a solution to existential safety, nor does it undermine the importance of existential safety in some surprising way. 

I.e., I agree.

we are so unprepared that the existing primordial norms are unlikely to matter for the process of settling our realm into a new equilibrium.

I also agree with that, as a statement about how we normal-everyday-humans seem quite likely to destroy ourselves with AI fairly soon.  From the OP:

I strongly suspect that acausal norms are not so compelling that AI technologies would automatically discover and obey them.  So, if your aim in reading this post was to find a comprehensive solution to AI safety, I'm sorry to say I don't think you will find it here.  

For 18 examples, just think of 3 common everyday norms having to do with each of the 6 boundaries given as example images in the post :)  (I.e., cell membranes, skin, fences, social group boundaries, internet firewalls, and national borders).  Each norm has the property that, when you reflect on it, it's easy to imagine a lot of other people also reflecting on the same norm, because of the salience of the non-subjectively-defined actual-boundary-thing that the norm is about.  That creates more of a Schelling-nature for that norm, relative to other norms, as I've argued somewhat in my «Boundaries» sequence.

Spelling out such examples more carefully in terms of the recursion described in 1 and 2 just prior is something I've been planning for a future post, so I will take this comment as encouragement to write it!

To your first question, I'm not sure which particular "the reason" would be most helpful to convey.  (To contrast: what's "the reason" that physically dispersed human societies have laws?  Answer: there's a confluence of reasons.).  However, I'll try to point out some things that might be helpful to attend to.

First, committing to a policy that merges your utility function with someone else's is quite a vulnerable maneuver, with a lot of boundary-setting aspects.  For instance, will you merge utility functions multiplicatively (as in Nash bargaining), linearly (as in Harsanyi's utility aggregation theorem), or some other way?  Also, what if the entity you're merging with has self-modified to become a "utility monster" (an entity with strongly exaggerated preferences) so as to exploit the merging procedure?  Some kind of boundary-setting is needed to decide whether, how, and how much to merge, which is one of the reasons why I think boundary-handling is more fundamental than utility-handling.

Relatedly, Scott Garrabrant has pointed out in his sequence on geometric rationality that linear aggregation is more like not-having-a-boundary, and multiplicative aggregation is more like having-a-boundary:
https://www.lesswrong.com/posts/rc5ZKGjXTHs7wPjop/geometric-exploration-arithmetic-exploitation#The_AM_GM_Boundary

I view this as further pointing away from "just aggregate utilities" and toward "one needs to think about boundaries when aggregating beings" (see Part 1 of my Boundaries sequence).  In other words, one needs (or implicitly assumes) some kind of norm about how and when to manage boundaries between utility functions, even in an abstract utility-function-merging operations where the boundary issues come down to where to draw parentheses in between additive and multiplicative operations.  Thus, boundary-management are somewhat more fundamental, or conceptually upstream, of principles that might pick out a global utility function for the entirely of the "acausal society".

(Even if the there is a global utility function that turns out to be very simple to write down, the process of verifying its agreeability will involve checking that a lot of boundary-interactions.  For instance, one must check that this hypothetical reigning global utility function is not dethroned by some union of civilizations who successfully merge in opposition to it, which is a question of boundary-handling.)

This is cool (and fwiw to other readers) correct. I must reflect on what it means for real world cooperation... I especially like the A <-> []X -> [][]X <-> []A trick.

I'm working on it :) At this point what I think is true is the following:

If ShortProof(x \leftrightarrow LongProof(ShortProof(x) \to x)), then MediumProof(x).

Apologies that I haven't written out calculations very precisely yet, but since you asked, that's roughly where I'm at :)

Based on a potential misreading of this post, I added the following caveat today:

Important Caveat: Arguments in natural language are basically never "theorems".  The main reason is that human thinking isn't perfectly rational in virtually any precisely defined sense, so sometimes the hypotheses of an argument can hold while its conclusion remains unconvincing.  Thus, the Löbian argument pattern of this post does not constitute a "theorem" about real-world humans: even when the hypotheses of the argument hold, the argument will not always play out like clockwork in the minds of real people.  Nonetheless, Löb's-Theorem-like arguments can play out relatively simply in the English language, and this post shows what would look like. 

Thanks!  Added a note to the OP explaining that hereby means "by this utterance".

Hat tip to Ben Pace for pointing out that invitations are often self-referential, such as when people say "You are hereby invited", because "hereby" means "by this utterance":
https://www.lesswrong.com/posts/rrpnEDpLPxsmmsLzs/open-technical-problem-a-quinean-proof-of-loeb-s-theorem-for?commentId=CFvfaWGzJjnMP8FCa

That comment was like 25% of my inspiration for this post :)