Often, the Vatican will purposely leave some rules vague (because there maybe some wiggle room, or some situations that it is okay, or because there are cultural reasons that it would be acceptable, etc.) In cases such as this, the Bishop has the final say so. It’s actually surprising how much say-so a bishop has for what goes on in his diocese liturgy-wise.
When the Bishop has not made a ruling on a particular question in regards to the Liturgy, the pastor would be the next choice. He is the pastor of that parish, and therefore (hopefully) knows the needs of his parishoners in regards to the Liturgy.
However, it is all Catholics’ Liturgy, and therefore all Catholics should be prepared to attack severe liturgical abuse when it occures (emphasis on severe. There are some minor liturgical abuses that become nitpicking, makes us miss the point of the Mass, and/or causes more problems than benefits). Therefore, the laity must, at the same time, understand the liturgical documents.
While there is a heirarchy, it is not a strict heirarchy (the laity should approach the priest and, if necessary, the bishop if the priest is doing something such as adlibing the words of Consecration). There are other times when it is best just to accept the pastor’s or bishop’s ruling or dicision something, even if we think it goes against liturgical law (in minor things, of course, and not those of a serious nature).