I have read through the GIRM several times, but I know enough to know that I know very little. So I don’t know if it is correct that any given priest has the authority, on his own whim, to simply decide in the parish he resides in, to say Mass ad orientem.
But we seem to be stuck on authority, and there appears to be a construct of a pyramid of authority from the Pope, to the dicastery, to the conference of bishops, to the bishop, to the priest, with the laity assigned to suck it up and “the faithful would conform their wills to ( if they desire to practice Virtue) would be to the will of God granted through Rome.”.
But let’s step back a minute; last I recall, one of the titles of the Pope is Servant of Servants. Which would lead one to consider that those ordained under him are also servants.
There has been much discussion elsewhere (and by that, I don’t mean in these forae - I mean among liturgists) as to the issue of ad orientem. But the fact of the matter is that while Rome may grant options, and the OP may be of the opinion (along with others) that the laity have no clue about liturgy, being sheep like and all that, that the shepherd (here, the parish priest) is going to show them what it is all about.
I would submit that unless and until Rome decides that ad orientem will be the rule and not the option of the exception, that the priest’s primary duty is to the salvation of the souls under his care, and that does not, in most instances, mean that he has carte blanche to use whatever option he feels most suits what he thinks they need as a spiritual direction.
I have been going to Mass ever since the OF was promulgated, to ad orientem Masses (with the exception of those at my local Trappist abbey - where the monks are in choir on one side of the altar, and the celebrant faces those laity who are on the other side. So about once a month I attend an EF that is a mixture of ad populum and ad orientem; but for the laity, ad populum as is just about every other Mass in the archdiocese.
For a period of time I attended a parish well out of the metropolitan area. In Oregon, we stand during the Domine Non Sum Dignus through the reception of Communion; but the pastor there seemed to think he had a different option (actually, several) and he had split that parish quite badly; people were driving 30 to 45 minutes to another parish 15 to 30 miles away.
It did some really serious economic damage to the parish, not to mention the social and spiritual impact it had on the parishioners who left, and those who remained behind.
The priest was moved. Last I heard, there still had not been anywhere near a complete healing the rift caused.
The point I am trying to make is that the priest is there to serve; and there are times he needs to change the directions of the parish; and there are times that wisdom would indicate that he should not. It is all well and good to claim “this is better liturgy” and “the laity need to conform their wills to God and Rome”. But what I am getting here is that there is a presumption the priest knows better than everyone else (which includes the bishop) because he a) has an option (which has now been elevated up to a right) and b) he knows better than the laity what their spiritual needs are.
It may well be that the priest has the option ad orientem and his bishop cannot say otherwise (my guess is that is not so, but it is only a guess based on 40 years of observation - or thereabouts), but that is more about liturgical law and Canon law than anyone here has shown an expertise about. In other words, the legal ramifications seem to be outside any expertise so far exhibited on either side.
But even presuming that the legal issue is that no one can tell the priest whether to use this option or not use it, only the unwisest and least experienced would say "Go for it Bro! (or Padre! if you will). And I say that having seen more than one parish torn asunder by a clericalist; the above example is only one of several I have witnessed.
Seminary may teach any number of things, and that is knowledge. Ordination, however, does not automatically grant wisdom, and wisdom and knowledge are most definitely not the same thing.