Congratulations! I’m sure so much love and heart went into this endeavor!

Thank you! To answer your questions:
-In the preface, I recommend study of the Graduale Novum, but this is intended as a book for the parish schola, not the expert or aspiring semiologist. It makes no claim to be a duplex or triplex edition, which would increase the page count by 50% or more.
-It is intended for the traditional Latin rite. In the the US, scholas singing full Gregorian propers for the TLM vastly outnumber those singing for the novus ordo. There is a wariness of change to be sure, but there are rather few places where the Solesmes method is followed faithfully. I think many choirmasters and cantors have never read the Rules for Interpretation in the front of the Liber, and plenty of Solesmes enthusiasts don't even know the ictus placement rules. The smartphone apps have increased awareness and appreciation of the chant repertory, but they have also added to the rhythmic confusion, with incorrect pauses at bar lines and uneven note values not characteristic of the Solesmes method. Those who neither understand nor follow the Solesmes method themselves are in no position to criticize me from departing from it in this publication!
-The episema is used for the ordinary long note, which the Cardinians call the normal syllabic value, both in isolation and in composition. My notation is geared toward singers transitioning from the Solesmes editions, and the rhythmic signs are used in a somewhat analogous way: a note with the episema is longer than the plain note, and a dotted note is longer still. Already in 2021 I requested the addition of gregorio glyphs for the descending tremula/inverted quilisma and upper auxiliary notes mirroring the initio debilis glyph, but as far as I know, the developers have not implemented either feature, hence my use of white/hollow notes. I have also used a white/hollow note to show a weak beginning note that is altogether absent in one or several reliable MSS—although one must still have recourse to the MSS or at least a triplex edition to determine how it is actually notated in the sources that do include it. (The critical apparatus for the Novum indicates lots of initio debiles notes that can't be discerned from the triplex notation alone.) Are there other deviations in my edition that aren't used by Turco or Stingl? I can't think of any.
-It is intended as a practical book to sing from. Again, I recommended the Graduale Novum to those who want a study edition. Incidentally, there may be a few solo cantors capable of doing so, but I'm not aware of a schola anywhere in this country singing complete Masses directly from the Novum, not a single one.
-Everything included in the hardcover edition will probably also be available for free on my website. That makes the physical book redundant for those who like singing from the PDFs, but there are many who prefer a book. Are people not still purchasing the other chant books? We still pay $70 for a new copy of the Liber Usualis or $55 for the 1974 Graduale Romanum, both of which can be downloaded for free, or nicely formatted individual chants compiled into booklets. The hardcover will not be ready for at least a year, maybe several, maybe never, which is of no use to anyone that wants a printed and bound edition to use immediately.

The distropha and tristropha retain the same appearance as in the Vatican edition, which is surely "still a notation that is recognizable as something we have become accustomed to over the last 150 years." Laon 239, on which my edition is most closely based, writes the short strophicus notes as ordinary puncta, as do most sources other than those of the St. Gall school, or a pes if not unison.

When 80% of the notes have a marking to lengthen them, you just ignore the markings. I appreciate the desire for nuance, but there’s a practical limit, and this is a bit much for me, I’m afraid.

If you are interpreting the long notes as nuances rather than 2:1 proportions, or ignoring the markings, you're not singing from the edition as it's intended to be used. Please see the table of note values on p. xii. I haven't received my own copy yet but anticipated a problem with the Amazon cover. The previous volumes have accepted the same cover PDF as Lulu, but not this one, for whatever reason unknown to me.

I now have the urge to mark the short notes, just to be sure not to miss them.

I had the exact same thought.

One idea is to create a neume that is twice as wide, which can easily be done with a font app. I am a big proponent of hybrid notations. If you want me to draw an example, I can.

Thanks, all, for your comments. Using the dot instead of the episema wasn't a viable option because of spacing issues with the compound neumes. One could use the space between notes to indicate length, as the Graduale Renovatum does, but that sometimes results in ambiguities about the value of final notes of groups (of course, the same could be said in some instances about the adiastematic neumes). Another possibility is to use white notation for the short notes, but that also causes some complications in "partially cursive" elements, and then there would be the problem of how to notate anything that's extra-short, i.e., the upper auxiliaries and passing tones mentioned above, until the developers have added the requested glyphs. Changing the spacing of 80% of the notes (to use James's estimate) or making 20% of the notation white would be a much more extensive alteration of the Vatican edition or neography than the addition of the rhythmic signs and, in my opinion, would be undesirable. What other means do we currently have at our disposal to differentiate long and short note values?

As for using modern notation, I'm not at all opposed to that, but I won't be the one to typeset a whole book that way. Bear in mind that modern notation will nearly double the page count. As for the notes of double width, again using the 80% estimate, we would be increasing the page count considerably because of the space needed for the wide noteheads. What would a long pes or clivis look like with double-wide noteheads? A long porrectus? I have 387 pages of chant, and another 40 pages for front and back matter and section separator pages, 427 pages total. Triplex notation would increase the page count by nearly 200 pages. Double-wide notation might increase it more than 200. Modern notation would increase it by 300 or more. Modern notation and adiastematic neumes in a triplex format could result in a book of more than 800 pages. To what benefit? Let me reiterate that this is intended as a practical edition for the use of a parish schola to sing according to a mensuralist interpretation, not as a study edition for semiologists.

I uploaded a new cover to Amazon last night, which should be live shortly. It is very strange that the required dimensions for the file size don't match Amazon's own print cover calculator. Amazon is cheap and quick, even without Prime, but you get what you pay for. The Lulu print quality is cleaner, darker, and better. Lulu does offer a global distribution option, but it jacks up the price another $8, not a penny of which makes it down to me.

Remind me, what does the original notation use, that is crystal clear about the four or five lengths: why can't we use that?

Remind me, what does the original notation use, that is crystal clear about the four or five lengths

Mostly the punctum for the short (8th) and uncinus/tractulus for long (quarter). The mora vocis (half) is unnotated but added at the end of sentences or phrases. A rounded initial stroke is used for shorter notes (16th), or the addition of c and/or t recognized from context. What about the dotted quarter? I have used it most typically when an already long note with an additional t is followed by a single short note or a group of three shorts.

why can't we use that?

Well, the adiastematic punctum is a round dot, which doesn't exist in the square notation. I'm not inclined to clutter the square notation with the letters c, t, a, etc., when an episema or augmentation dot will do just fine. The neographical initio debilis form corresponds rather closely to Laon's rounded initial stroke. Even where they're in perfect agreement about the note values, Laon and St. Gall don't always use the same forms. Example 1: pes subpunctis (short). Laon writes torculus+punctum; St. Gall, pes+two puncta. The Vatican notation follows St. Gall. Example 2: short climacus: Laon writes three puncta, or two puncta+uncinus; St. Gall writes virga+two puncta, or virga+punctum+tractulus. Vat follows St. Gall. Example 3: unison tristropha. Laon writes two puncta+lineola (this terminology is disputed; some call it a tractulus); St. Gall writes a form that looks like an apostrophe, sometimes with an episema on the third note. Vat follows Laon.

For comparison, here is part of a melisma from the gradual Universi for the First Sunday of Advent.

Line by line:
1. Graduale Novum
2. Graduale Renovatum
3. Graduale Lagal
4. The Proper of the Mass in Proportional Rhythm
5. dots
6. shorts white
7. longs white

I like 5 and 7 the least. Which do I like best? 4, and it's what I went with.

I can do this kind of stuff easily enough with gregorio, but why? If we want to say a rectangle is long and a square or diamond is short, we might as well make them breves and semibreves and go back to Medicaean notation. Like modern notation, I'm not necessarily opposed to the idea, but I won't be the one to typeset a whole book that way.

I can’t remember who said it where but someone visiting the abbey described a conversation about interpretation wherein a monk of Solesmes described the post-Cardine long notes as all long and drew in the air an episema over the entire chant. I don’t necessarily hear it in the occasional recording of Dom Claire to which I listen. But it’s an interesting thought. (Of course, it’s still not proportionalism in a 2:1 ratio, but it’s not short-as-default either.)

Oh yes. I reread that article in the last year!