Can I try to help?
I did study formal theology for many years and taught as well. The rubrics of the mass, including the words in the prayers, are part of Church law and are governed by liturgical law.
The theology of the filioque was formally endorsed by the Church during the Middle Ages, but it was believed for centuries, back to St. John the Evangelist who is the first to propose it.
That being said, a Pope is not bound by liturgical rubrics, just as he is not bound by Canon law. He is bound by faith. Therefore, the elements of the Sacrament of the Eucharist, must be present. Those are part of the faith of the Church.
How he celebrates the mass and what words he omits or inserts are up to his discretion, as long as it does not deny the faith or change the essence of the Liturgy.
For example, there is a formula for Baptism used among many Protestant churches that says “I baptize you in Jesus’ name”. That’s a change in the essence of the baptismal liturgy. You have to baptize in the name of Father, Son and Holy Spirit.
How ever, if the Pope said, “I baptize you in the name of the Logos, and his eternal Father, and the Holy Spirit that flows from their love” or some such wording that includes the entire Trinity and makes it clear that he is baptizing in the name of the Trinity. This certainly a strange way of saying it, but it fulfills the requirement for the Baptismal liturgy. The formula is trinitarian.
If the pope leaves out the filioque, but accepts the theology of the filioque, he is not leaving out anything that is absolutely necessary for the validity and leceity of the Eucharistic liturgy. As I said, he bound to preserve the faith, but not bound by liturgical and canon law. He is the only Catholic who is not bound to such laws.
I hope this helps. By the way, those words of baptism that I wrote above, are an example that I made up, I’ve never heard anyone use it. Actually, I kind of like it
Don’t lynch me, just adding a little levity to an otherwise toooooo heavy thread which began with a very simple qeustion.