Once again as repeated a number of times on these forums, this is letter does not have the effect of law in the Church. The decision on this matter is up to the local ordinary. If he allows the practice, there is no futher aurgument.
Not true. In fact, the exact opposite is true.
The letter does indeed have the effect of law in the Church. I suggest you read canon 16 which contradicts what you posted above.
Canon 135 §2. Legislative power must be exercised in the manner prescribed by law; that which a legislator below the supreme authority possesses in the Church cannot be validly delegated unless the law explicitly provides otherwise. A lower legislator cannot validly issue a law contrary to higher law.
Can. 360 The Supreme Pontiff usually conducts the affairs of the universal Church through the Roman Curia which performs its function in his name and by his authority for the good and service of the churches. The Roman Curia consists of the Secretariat of State or the Papal Secretariat, the Council for the Public Affairs of the Church, congregations, tribunals, and other institutes; the constitution and competence of all these are defined in special law.
Can. 361 In this Code, the term Apostolic See or Holy See refers not only to the Roman Pontiff but also to the Secretariat of State, the Council for the Public Affairs of the Church, and other institutes of the Roman Curia, unless it is otherwise apparent from the nature of the matter or the context of the words.
Can. 16 §1. The legislator authentically interprets laws as does the one to whom the same legislator has entrusted the power of authentically interpreting.
§2. **An authentic interpretation put forth in the form of law has the same force as the law itself **and must be promulgated. If it only declares the words of the law which are certain in themselves, it is retroactive; if it restricts or extends the law, or if it explains a doubtful law, it is not retroactive.
The Holy See decides what happens/doesn’t happen at Mass. It’s not up to the local ordinary because the local ordinary (while certainly being the chief liturgist of his diocese), does not have the authority to over-rule the Holy See. It is the role of the local ordinary to implement the liturgical laws which the Holy See alone has the authority to change.