Selecting an optional memorial or a votive mass will change the "Presidential Prayers." In other words, it will change the prayers prayed by the priest alone i.e. the Collect, prayer after communion, etc.
what are those who don't know the proper meaning of the word "liturgy" supposed to conclude from all those options?
when you mention "the Missal" as one of the two pieces of bread, I think you are referring to the "presidential prayers" which come from the Missal; and the other piece of bread, which is the Gradual, should match those?
What is also common for Memorials ... is for the First Reading and Psalm to be chosen from the ferial lectionary, and the Gospel from the Proper of Saints
Of course it's licit: GIRM option 4: "alius cantus aptus".
There are over a dozen chants to choose from between those two commons when one considers the Graduale Romanum, the Simplex, and the Missal, so using the ferial chants might be the easiest option; and, if the idea of singing the chants instead of hymns is new, it might be the most pastoral.
what would be the point of marking the memorial of St. Peter Claver with OT23, or the current time?
The multiplicity of options makes it difficult to plan music.
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