I’m continually surprised that some people in such a creative endeavor like making music have such a strong drive to find justification in legal guidelines. Musicam Sacram is the the “last” law. No, it’s not; the GIRM supercedes it. What about SttL? Forget it; those scaredy-cat bishops didn’t even submit it to Rome.
My impression of music is that the great leaps of inspiration come from using law as a framework, then expanding from there to explore new possibilities. Organum from chant. Polyphony from organum. Fugue and counterpoint from polyphony. Why one would want to be confined, as an artist, even an artist for God and the assembly, in principle to the exact prescriptions of the law is beyond my understanding.
On the other hand, we’ve had a rather thorough treatment here of some of the documents in question. I’ve yet to find a conservative website so willing to delve deeply into what we’ve examined here.