[This is all about the traditional Roman Rite, and not about the Novus Ordo.]

The 1908 Vatican Graduale contains nine numbered rules under the above title, providing for how a cantor, cantors, and chorus, and the priest, are to be assigned the various parts of the ordinary and the proper. The same title appears in the Liber Usualis: in an English version of that it's called Rubrics For The Chant Of The Mass.

I’d like to know: is there a predecessor for these rubrics? or were they composed for the Vatican Gradual? How normative are they?

The 1963 Liber’s rubrics contain additional paragraphs proposing: ways to extend the Offertory (e.g. with psalm verses or with the more ancient Gregorian chants); how to not sing the Benedictus; and how to extend the Communion rite music when there are “other communicants”.

I’d like to know : are the Liber’s additions just suggestions by Solesmes? Or have they the same status as the rubrics in the Vatican edition?

In general these rules refer to “one, two, or even four cantors” for various things and with various asterisks. I feel sure that the observance of asterisks is not a religious obligation! But I’d like to know: is singing by one cantor forbidden, or discouraged, or neutral? Are there customs (or rules!) about how many singers and who sings what, recorded elsewhere?

Lastly I see nothing there forbidding mixed (treble with tenor or female with male: ultimately, octave organum) chanting, but I’ve certainly heard pushback against it. Is it anywhere forbidden or discouraged?

(Tra le sollecitudini doesn’t reach any of the above, although it approaches some of them distantly.)

These are all practical questions, albeit not hard pressing, for our years-standing schola of three women and three men.