I would do exactly the same thing, but use the solemn mediation (like for the Magnificat at Vespers)

"Richard Rice's Communio book has three custodes and a change of clef and (seems to) propose chanting the verse untransposed, which establishes a new final and requires a leap of a minor seventh (from mi to high re) to resume the antiphon."

While the custoses may be confusing, I am certainly not proposing such a leap (though it is possible a confusing version that might suggest it may be floating around somewhere). The attached shows clearly (enough) that I propose the same final for antiphon and verses on mi, with a leap of a (very manageable) third from end of verse to beginning of antiphon. This is the same solution given in Versus Psalmorum et Canticorum. Alternatively, the psalm tone could be written using the same clef and transposition as the antiphon, since it does not involve the fa-cum-ti-flat. I provide that as version 2 below (but please check my search and replace didn't cause problems).

I did sing this (Sept 13, Wk 24 in the NO) and used the reciting note re and tone you plan on using. I also made the correction of the note at the beginning of the last word of the antiphon as done in Gregor und Taube. I have discovered their readings to be highly convincing but did not agree with their choice of psalm tone in this instance.

Er...

Well...

By restored I mean that in the older books, such as the Liber, mode IV recites on the fourth above the final, but in in more recent books reciting on the third above also appears. It's my understanding that this change is a restoration of a much older practice.

I reckoned that the melody is written transposed when it's decided that the fourth below the final (on -num of Dominum) should be flat (i.e. an augmented fourth). It's my understanding that "low b flat" accidental was avoided in the earliest diastemic notation (and I cannot see it explicitly in the two linked MS above). In the transposed mode, the fourth below is already augmented. However, I'm happy to be corrected on this point.

Since in the transposed mode the fifth above is consequently diminished, reciting the verse on mi sounds wrong unless you sing fa sharp. (If you sing Richard's second example above, reading the notes literally, you'll hear what I mean.) So in Richard's book the antiphon is written transposed but the verse tone is written untransposed, but (as I now understand) the custodes indicate that the pitches should remain transposed: the recitation is written on la, but is supposed to be sung a fifth higher at mi. And then of course what is written as ti should be sung as fa sharp!

Great. Except that the augmented fourth below sounds so cool that I want to avoid singing the perfect fifth above. My ear wants that to be "still flat", that is, diminished. So, I proposed chanting the verses at a third above the final instead of a fourth, for then the fifth above is avoided altogether.

Indeed I described the Office reciting tone; but @salieri rightly proposed using the solemn tone instead.

For the mode IV psalmtone, the alternate recitation on sol, rather than la, appears in recent books for the Divine Office, and is called IV*. The range and character of the communion antiphon certainly suggests its use here, transposed to recite on re.

Is there something there that tells which verse tone to use? I would say, reading the neumes, that's a verse tone i have never heard before -- but if it indicates the reciting pitch, I cannot tell how.

Is there something there that tells which verse tone to use? I would say, reading the neumes, that's a verse tone i have never heard before -- but if it indicates the reciting pitch, I cannot tell how.

Like other liquescent notes, the cephalicus is ambivalent and can denote either "augmentative" liquescence on a single pitch or "diminutive" liquescence on a secondary pitch. I have used parentheses for secondary pitches.
cephalicus, pes, four virgae, cephalicus, pes, cephalicus, pes, tractulus: cephalicus, pes, four virgae, cephalicus, pes, cephalicus, tractulus=high-(low) low-high high*4 high-(low) low-high high-(low) low-high low: high-(low) low-high high*4 high-(low) low-high high-(low) low
If you have the tones for the Gloria Patri at the introit handy, you'll see that this corresponds exactly to mode 4 and none of the others.

Is there evidence for a historical mode IV reciting tone a third above the final? There seems to have been some confusion in this discussion about modes III and IV. It is difficult to know for sure what is intended in this edition because of the clef change and lack of custos. Taken at face value, the untransposed psalm tone begins a step lower than the final of the transposed antiphon. Fine and well, but then there's a seventh between the end of the verse and the repetition of the antiphon! In my opinion, the solution of the Graduale novum and Stingl is best: notate the antiphon with an F clef and B-flat at Dominum. Then the untransposed introit psalm tone works fine without modification. The objection about the B-natural in the verse following the B-flat in antiphon is later theory. The earlier scribes knew no more about Guidonian hexachords than they did about square notes, Solesmes rhythmic markings, equal temperament, or the A=440 pitch standard - modality and tonality, yes; theory, no. I've attached the psalm tone for this verse with two transpositions.