The GIRM uses “usually” (de more) to state a norm which admits of occasional legitimate variation, for example:

52. [re: the Kyrie] Each acclamation is usually (de more) pronounced twice, though it is not to be excluded that it be repeated several times, by reason of the character of the various languages, as well as of the artistry of the music or of other circumstances.

61. The Responsorial Psalm should correspond to each reading and should usually (de more) be taken from the Lectionary.

There is no indication or suggestion that legitimate variations are inappropriate.

The rule of thumb not to be forgotten is that exceptions should not become norms.

Supplicatio Agnus Dei a schola vel a cantore,
populo respondente, de more cantatur, vel saltem elata voce dicitur.

It does not seem to support your interpretation Marajoy, but it is a little vague. If the authors had wanted to group the people with the cantor, it could have been stated more clearly. The commas suggest that the OR is between the choir and the cantor, with the people responding.

“I would have a difficult time accepting that the intent of IGMR is to suppress choral settings of the ordinary....”

An odd way to put it. It is perfectly clear that the GIRM does not “suppress” choral settings of the ordinary. The GIRM does however reflect that fact that the Church now gives priority to the singing of most of the ordinary by the people and not exclusively by the choir. If you have a difficult time with that, then you are not simply at odds with a document, you are at odds with the Church’s vision of the typical celebration of her divine worship.

“We cannot read these documents in a vacuum, as if the liturgical life of the Church began in 1969.”

Absolutely, just as we must avoid devolving into personal interpretation.

“Rather than debate what is proper based on the placement of a comma ...”

In other words, we should ignore any clarification punctuation provides if it undermines our position.

“I suggest we view the situation in light of tradition ...”

... and override the intent of the instruction in favor of my historically-informed opinion.

And where does theology enter in? And if the Church changes her tradition, what then?

The question of a choral Sanctus is particularly nuanced and much debated.

The GIRM’s intent is to set forth the typical celebration of the Roman Rite, indicating norms and exceptions. Elevating the importance of congregational singing of the ordinary to the level of a norm is not the same as banning the Masses of Palestrina, Victoria, Byrd, and the like, but grants them a secondary place in the typical form of the celebration. I see no indication that Pope Benedict has any problem with that, and I see no indication that he wishes to alter what is typical and what is exceptional in this regard. I would be happy to be corrected with supporting documentation.