Thanks for all of this, Aristotle!

This was the first time that I've taken a close look at the Kyriale Simplex. I was always under the impression that its contents consisted of altered, simplified melodies from the Kyriale, but I see now that instead it simply is a collection of melodies which themselves are already on the easier side of things… nothing's altered. (Am I right?!)

(For those who might be following this thread, here is a corrected link for the Proulx Gloria Simplex. Thanks again, Aristotle, for all of these links.)

Others?

A related question here (and a shameless thread bump): Is there any guide as to which Memorial Acclamation ought to be used when? Or is it entirely a matter of preference?

To somewhat answer your original question-
I do 6 settings over the course of a year: Advent, Christmas, OT1, Lent, Easter, OT2

I've thought about switching halfway through the second Ordinary Time, but it seems arbitrary- which is one of my least favorite liturgical feelings.
This means, though, that you need a seriously hardy setting that won't get boring for such a long stretch of time.
My (Episcopal) parish seems to really like the Deutche Messe adaptation in the 1982, so we use that.

Back to chant...
It is by no means required or mandated, but the Vatican has a list of suggested chant settings for particular seasons:
http://antoinedanielmass.org/kyriale/vaticannorms.pdf

Mark M. - Agnus Dei ad lib #2 is a 19th century composition by Dom Joseph Pothier. See this thread.

Sadly, it's not a perfect world, although I wish it were. But you are right that (much) more frequent changes in the ordinary of the Mass is highly desirable. For parishes mired in the same ol' same ol' - week in and week out (only rarely changing except for special celebrations or seasons) - an incremental approach might (unfortunately) be necessary, both in terms of frequency of change and in terms of the variety of ordinaries (especially when only one or two have been used in the past). But I would make the transition as swift as possible.

Chuck's observation is very salient. Lamentable this world may be, and dove-tailing from another thread, we ought not to forget that Pius X's revolution eventually insisted the ordinary ought to be owned by the faithful (all things being equal, arggghhh.) MS puts that into reality from the Pater Noster on down the line.
And one of the ironic points that both wings of liturgipraxis agree upon is the "by memory" thing. Well, in MJM's perfect world we'd all have to have photographic recall and pretty high IQ's. Again, remembering many talks/essays by Mahrt, this memory process takes decades. And, of course, the propers were essentially carried forth by rote learning and memory by children in monasteries by the systematic calendaring and repetition of those scriptural excerpts over centuries' time.
The ironic part: the ex-communicant priest Mathew Fox trumpeted the "lose the hymnal, reduce the repertoire" mantra at NPM two decades ago. And that's been resurrected in some threads over at the PTB recently as well.
The path of least resistance has commanded us to sing "Happy Birthday" for someone every darn day to the same darn melody. It doesn't lessen the essence of joy we should feel for the presence of the birthday boy/girl.

Even if you're not using Gregorian Ordinaries, I think the wisdom of the Vatican's seasonal/feast-day approach to Ordinary settings is an excellent example to follow.
It also aligns a traditionalist practice with a contemporary/progressive "liturgical season mood setting" conception.

We will have three mass settings, instead of the current 4 settings, for a one year period that begins with Advent. After the year is up, we can use what we like. I normally change seasonally. Regardless of other settings we may choose at the end of the prescribed year, the English chant ordinary from the missal, and the Latin ordinary will stay with us.

Gloating is unbecoming, Dad! ;-)
But our "Garage Schola", The Gregorian Schola of St. Francis (all 3rd Order guys) were there in force, so they were quite taken by the ICEL Gloria (XI) and JMO's "Sherwin." So, there too!

Mix it all up a bit.

The are 4x Kyrie which I have identified and intend to teach to my schola

Kyrie XI (Orbis Factor) is a little elaborate but quite manageable.
Kyrie XII is simple, and really seems to be a forgotten gem of chant.
Kyrie XVI is the easiest one used in both Missa Primitiva and Missa Jubilate Deo
Kyrie XVIII is similar and not hard to sing at all.

I would teach the following 3x Glorias

Gloria VIII (De Angelis) is a beautiful one and easy to learn.
Gloria XV - not as jubilant as Gloria VIII
ICEL Chant Mass for the Assembly (English version of Gloria XV)

Perhaps these two are best contrasted by using Gloria XV (or the ICEL version) on Ordinary Sundays and Gloria VIII on Feasts or Special occasions.

Sanctus XI (Orbis Factor), again is a little elaborate but still manageable.
Sanctus XVIII is the staple used by all, and should be used regularly.

Agnus Dei Ad Libitum II - I would alternate this with XVIII during the year.
Agnus Dei XVIII is another staple piece, I would stick with this during Lent and Advent.

And of course, there are good settings in Latin and English for congregational singing so don't be afraid to use these as well.

Just to give you an idea of what I would do, here is my own vision for the Kyrie:

Advent and Lent are the penitential/preparatory seasons, so stick to the simpler ones during these times, and focus more on chant masses. I would use Kyrie XVI and XVIII during these seasons and Kyrie XII on Laetare and Gaudete Sundays to indicate the softening of the disciplines, much in the same way that Rose is a softening of Purple vestments. I would then use Kyrie XII more often during Eastertide and Christmastide. I would then use all four of the above throughout Ordinary time.

This is assuming that I only use the Chant Kyrie. I would in fact make use of harmonised or polyphonic Kyries as well throughout the year, as well as different versions from various mass settings. Mass St Boniface and Mass in Mi are two which immediately spring to mind.