Would you say that this is a mainstream idea amongst Catholic THEOLOGIANS?
Well, depends what you call 'mainstream'. If you mean mainstream as the most easily accessible writings (especially on the internet) then decidedly not.
If you consider mainstream as the thinking of the orthodox Catholic academia [lowercase o capital C] in the major universities, as well as the opinions of the important members of the Vatican curia [basically the people who matter because they govern the Church], then the answer is 'basically, yes'. These people have can read the original documents and put them in their broader context. However, their writings are not at all easily accessed, and are frequently written in French, German or Italian, quoting on Greek or Latin texts. This teaching on the Pope purports that the understanding of the papacy that St. Gregory the Theologian, the medieval popes of and immediately after the Gregorian reforms of the 11th century, Vatican I and Vatican II are consistent with each other - when taken in their proper context. And I know of no Catholic theologian who purports that the Pope is an absolute monarch who can go against the tradition of the Church; the mainstream understanding of the papacy is that his authority is given to him to safeguard the tradition of the Church.
Then-Cardinal Ratzinger - among the most important Catholic theologians of the second half of the twentith century - is using this quote as a common thread to discuss the history of the papacy. In this quote he uses this understanding of the Pope and Christian tradition as a bulwark for the inherent liturgical Rites of a particular church, because they come from Christian tradition and should not be tampered with, since liturgy springs from the Church itself and cannot be altered without
I think that the sign of the cross IS of major importance. Look at the "Old Believers" in Russia. They were declared Heretics. Why? Because they focused on ONE THING, just like all other heresies do. They focused on HOW TO CROSS YOURSELF! So there's more to this than meets the eyes.
I also believe that there is a LOT of theological significance in the Cross and making its sign. If it can ward off demons, then we should take a deeper look at its meaning.
Oops, let's step back a bit. Yes the meaning behind the Sign of the Cross is definitely of major spiritual significance, and I don't want to give the impression that I don't think so. However, it is a fact that the Roman tradition does it one way, our tradition does it a seperate way, Old believers a third way, and the Coptic tradition reportedly do it the western way (I've never been to a Coptic church so I don't know how they do it). In the patristic era, the tradition supposedly began with simply a cross on the forehead, like the Latins do at the Gospel.
My view on this is that there are certain traditions that grow in a particular Church or group of Churches that are inherent to it. Its liturgy and symbolism are among the major traditions of a Church, and they should be allowed if they're not newfangled and arbitrary changes, if they have spiritual significance, and it they come from a long tradition of a real particular church.
Particular churches, when confronted by differences, can deny the validity of the other and become schismatic with each other, submit to the demands of one (a grave ecclesiological error) or learn to accept the differences while maintaining a common faith. Accepting the differences is in my view the best ONLY when the matters are not dogmas of faith, and it's the only one that can work in a Church where Roman, Byzantine, Coptic etc. rites are in union (though submitting to the demands of one - Latinization - has been tried, with horrible result). This is also the Catholic [capital C, meaning churches in union with Rome rather than simple "catholic"] view since the Catholic church purports to unite all the traditions of Christianity in union with each other and the Roman See. Only certain differences can be held as dogmas of faith; others - especially ones so rich in significance as the sign of the cross - must be mutually tolerated.
Now, I won't speak for you Orthodox. But aren't there old ritualists under the omphor of the Moscow Patriarchate? If so, do they use the two or three finger cross?
Anyway, I hope some of that makes some sense. I think it does, but I can't proof read it right now. It's getting late and it's time for prayer.