Surely, then you will find other documents to clarify the issue if it is so clear. Yet, the GIRM, does not prohibit the laity from raising their arms.
That doesn’t hold up to scrutiny.
There’s no prohibition against having a dancing monkey playing a squeeze-box during the psalm. That doesn’t mean it’s right to do it.
Liturgical norms do not say what-not-to-do. They say what is done. That’s how those documents operate.
If something is not described, then it is, by that very fact, not to be done.
I can go on forever listing things that the GIRM doesn’t prohibit. That does not mean they’re acceptable liturgical practice. It defies sense, and more to the point, it defies liturgical practice, to claim that just because there’s no specific prohibition against doing some specific thing, then it is thereby permitted. It’s utter nonsense to think that’s how liturgy works.
There is a very clear prohibition, going back to the documents of Vatican II (SC #22) which prohibits adding, removing, or changing anything. That’s plain enough; at least it should be plain enough.
I’ve already explained that the orans posture is a presidential gesture in the Latin rite. It is a gesture which is reserved to the priest because the priest presides at the Mass. The Church only needs to say it one time, that the laity do not assume the gestures of the priest at Mass.
There’s no sensible reason why saying it once in the GIRM should be insufficient, or that it needs to be repeated for every single rubric in the Mass.