Consider a Z13 analysis complementary to that in my first post. There, Zodiac’s repetitiveness suggested “slaves’ slayer” as a unique solution (with an unexplained but unsurprising letter swap) confined to his own vocabulary, exceedingly unlikely to thus emerge by chance (so highly statistically significant), but probable given previous texts, and sure to emerge if Zodiac hinted at it (and used a homophonic cipher, even with transpositions). Here, let us use a different approach, incorporating the full English dictionary, but constraining the solution based on previous ciphers and hints apparently provided by Zodiac.

We will impose quite a few constraints, some stronger than others, based on my understanding of Zodiac texts and discussions in this forum. These assumptions are not directly based on my first analysis, but it surely biased my approach. You may disagree with some constraints: they are gradually relaxed below, and my (Wolfram Mathematica) code is available if you wish to further test other constraints or combinations.

Constraints:
C1. Substitution cipher with transposition step j (no anagrams).
C2. Self-title (“I will not give you my name” + “My name is __ [Z13]”), so English dictionary words with no additional last names.
C3. No homophones (apparently Zodiac wanted this message readily deciphered) unless a letter immediately repeats.
C4. At least one word based on Zodiac’s repetitive and hinted vocabulary (envelope + letter + cipher words + their derivatives).
C5. Z340-style inversion: one word possibly spelled backwards.
C6. Local inversion: one pair of adjacent letters possibly swapped (deliberately or accidentally).
C7. j=-3 (ZF dots and other reasons; immediately follows if the cross-hair symbol coincidence with the Z340 ending applies here).
C8. Template: <six letters> + <redundant letter> + <six letters> (inferred from "sorry nO cipher").
C9. Each <six-letters> likely to be one word (inferred from "sorry nO cipher" with its surprising "sorry" + more likely given C1).
C10. The redundant letter is e (these reasons + Zodiac tried to help e.g. with multiplicities: many e’s most helpful for decryption).
C11. Message in minimal nonzero depth: at least one careless/deliberate use of the Z480 key, but only in a second homophone.
C12. Message in depth under key inversions: d>0 instances of the Z480 key reversed (taking e.g. Lambda=L, Phi=P, Delta=D).

Note that my previous analysis only assumed C4 (generalized to apply to the entire message there, and entirely relaxed below), so its outcome would become even more significant if supplemented with any other constraint. Both studies do not specifically utilize the two strongest (if correct) Zodiac hints:
C13. Likely to include "slave" or its derivatives (inferred from focus of otherwise mostly unoriginal Halloween card, and elsewhere).
C14. Likely to Include "slay" or its derivatives (inferred from the otherwise bizarre SLA letter).
You may think of other constraints. For example, many have focused on the symmetry of the cipher, and it would be surprising indeed if the solution had no special symmetry. The depth constraints (C11 and C12) can be explored with the Z480 key replaced by Z340, unless Zodiac prepared Z13 earlier (more likely: he made them together) or perhaps was trying (when Z340 was unsolved) to make it easier to decrypt Z13. These and other possibilities are not included in the present analysis.

Let us begin restrictively: combine all constraints, temporarily without utilizing C5 (word inversions incorporated only later). First, consider self-titles that Zodiac might fancy from his vocabulary: these presumably contain ‘killer’ or similar (here, six-letter) words such as slayer, bomber, robber, hitter, bullet, master, etc. We get the expected very deep (d=4) solution "slaves slayer", as well as its variations "slakes slayer" and "slates slayer", which are indistinguishable within Z13 but are much less likely to be correct (not entirely in the Zodiac vocabulary and far more nonsensical for Zodiac). Next, and at a much shallower depth, we find "dieted hitter" (d=1), "dueled bullet" (d=0), and that’s it. All the above solutions require a letter swap (C6).

Without guessing words, allowing instead for any English word + Zodiac word (or vice versa) yields 22 additional d=4 solutions, including unreasonable variations on the above ("slaves slaked", "slates slaves", etc.) along with other gibberish ("slopes slower", "sleeps selves", etc.), 54 new d=3 solutions (best I found are "sprees parted" and "novene movies"), etc. Completely relaxing C12 (i.e. allowing any d) we get in total 330 solutions, including more interesting ones ("unseen rusher", "speeks spades", etc.) at zero depth. Only 10 solutions do not require a C6 letter swap ("noreen corner", "stress street"…). Relaxing C10 (so the redundant letter is not necessarily e) adds some but not many solutions, for example nine new d=4 options ("slaves saliva", "spoils sophia", …). Similarly, incorporating word inversions (C5) does not qualitatively change the outcome, yielding no reasonable solutions.

Let us now relax C4 entirely, incorporating the full untrimmed English dictionary with no Zodiac-vocabulary restrictions. Starting as above (with C10 and C12 but temporarily not utilizing C5), we have 98 new d=4 solutions (the best of which are probably "slices slicer" and "sleets sleaze"…), 362 new d=3 solutions, etc. Note: "slaves slayer" uniquely emerges from the gibberish of the 123 deepest constrained solutions, although we did not restrict the English dictionary at all. Relaxing also C12 entirely, we get in total 1658 solutions, which is still a manageable number. I found a few more interesting solutions ("unseen buster", "snakes snares", etc.) at shallow depths, none of which makes nearly as much sense for Zodiac as "slaves slayer". One can continue to relax assumptions and study gradually longer (and less likely, if the above constraints are reasonable) lists. If you want to use my code to explore the above results, add/modify/remove constraints, etc., please send me a PM.

In conclusion, my interpretation of the hints left by Zodiac (some but not all may be disputed; see first post) and the properties of his ciphers lead to "slaves slayer" as the most likely and only sensible (in this context) solution to Z13, even without restricting the text to the Zodiac dictionary, and without explicitly invoking the major constraints C13 and C14. The frequent cipher multiplicities are key here, are intentional (otherwise homophones would render them much rarer, as in Z408 and Z340), and suffice for a unique cipher solution when combined with adequate constraints (as done here, or independently by resorting to the Zodiac vocabulary).