I thought I did answer your questions. But let’s clarify a couple of definitions first.
The “Ordinary” chants of the Mass are those chants in which the texts are invariable from Mass to Mass. The chants are, in the order they appear in Latin, the Kyrie (unless the rite of aspersion with holy water is used), the Gloria (Sundays, feasts and solemnities), the Credo, the Sanctus and the Agnus Dei. The texts of these chants never change. There are different melodies for these chants, with the same texts. They are more or less ornate as befits the occasion. There are 18 standard Mass settings to use according to occasion, that is these same texts but with melodies of varying complexity.
The simpler versions, are well within the reach of the laity (I posted above which those are). The most complex ones, are not.
The “Propers” of the Mass are variable texts. In Gregorian chant, a series of antiphons which are almost always psalm or Bible verses related to the theme of the day’s Mass (feast, season, Gospel of the day can all set the theme). These texts thus vary depending on the season, day, feast, etc. They are in order: the Introit (entrance antiphon), the Gradual (a psalm verse or two set to a very ornate melody), the Alleluia and its psalm verse, the Offertory, and the Communion antiphon. In general, except for a very few simpler introits and communion verses, these are too complex for the laity to master. If they attend only Sunday Mass using these texts, they’ll hear the same Propers only once or twice a year (a few of the chants are repeated in the year, not many). Dirty little secret: the monks have chant class at least once a week; that’s how they can manage a chant they only sing once a year. The laity would have to do the same.
The Simple English Propers are the
proper texts of the Mass, but set to simple settings. They should be as easy to learn as the simple settings of the ordinary. Thus the laity
could be induced to sing the propers, simplified and in the vernacular, instead of the usual three-hymn-sandwich. Theoretically at least.
To answer your question about the laity, they don’t have to do anything. They could be
encouraged to sing the simple settings of the ordinary, but would never sing the full Gregorian propers. They did in the past. That’s the choir’s job, and for the really technical pieces, a small subset of the choir called the Schola.
Note I’m putting this in a monastic context, as that’s the most usual place one hears the full Gregorian propers in the Ordinary Form, on a regular basis.
So, in a nutshell, your questions:
- See above
- No, Gregorian chant means the propers and the ordinary.
- Some substitutions are permitted for the propers but it’s not customary.
Follow-up:
- No the laity never have, and never would do, something as complicated as that offertory.
- Gregorian chant is the patrimony of the Church, so one doesn’t typically “mess” with it, but the Simple English Propers do what you suggest; it is new music, but with Gregorian modality.
- No.
- Not sure I understand the question.