You wrote, "It is identical save a couple of neumes." Therein lies the problem.

Much of the Latin chant corpus consists of adaptations, but they are true adaptations. Their composers adjusted melodic material composed for one set of words to make it fit (and not merely accommodate) another set of words. For an example from the "golden age" see the Offertory, "Iustorum animae," which is adapted from the Offertory, "Stetit angelus." Its composer, using the melodic material from "Stetit angelus," created a new composition built upon the text of "Iustorum animae." Compare also the Introit, "Gaudens gaudebo," composed a little more than a century ago by Dom Pothier, with the Introit from which it was adapted, "Vocem iucunditatis." You will see that Pothier employed the same method of adaptation used by the composer of "Iustorum animae."

Chants were never conceived in vacuo. They were built upon texts, even when they were built with pre-existent melodic material. Chant is ars recte loquendi. When one text is merely laid under a set of notes composed for another text, the new text is inevitably distorted. To prove my point I need only to point to the intonation in your adaptation of Gloria IX, where the preposition, "in" stands at the climax of the musical phrase, while "God" stands on the neume that is clearly preparatory to this climax.

May I ask you to take a look at Winfred Douglas's adaptation of this melody to Cranmer's translation of the text? You can find it on this site at

http://musicasacra.com/books/ordinary_of_the_mass_english1933.pdf

beginning on page 84. Douglas employed the methods of adaptation employed by the adapters of the "golden age," just as Pothier did when he composed "Gaudens gaudebo."
Consequently, one who sings the English text has no sense that the music he is singing was composed for another text.

You write: "The idea is to get the melody and text into the heads of the faithful so that a switch to the Latin here and there would not be such a big deal. Multi-year plan, of course."

Is it really appropriate to sing a distorted version of the English text in the liturgy regularly, for a year or longer, simply to teach people a Latin chant?

John Boe, sometime professor of music history at the University of Arizona, wrote a comparative study of "Anglican Plainsong Kyrials and their Sources." as his doctoral dissertation, called, "The Ordinary in English," It is a highly-informative work that everyone engaged in adapting Ordinary chants to English words can benefit from reading attentively.

Boe would probably be the first to say that his dissertation is dated. Still, I have seen no study of the adaptation of chant to English that is more insightful.

I am willing (along with many others) to make the judgment that a text is distorted when an insignificant preposition coincides with a phrase's melodic climax.

I don't know what he would have to say about Saulnier's work on modality.

I read Apel's book thirty years ago and found it wanting even then. His treatment of chant composition is not very insightful. I remember that he described Credo I as a cento. I suppose that description cannot be called incorrect; but it ignores the fact that the piece is based on three set-forms, as Mocquereau had demonstrated fifty years before Apel wrote his book. His quantitative approach to analyzing the techniques used in adaptation ignores the fact that some medieval adaptations had more artistic merit than others

I already have an adaptation of this i made 2 years ago...