How is the Bible above Tradition and everything, when it was put together by Councils which had no Bible to look to?
I think that your post ignores the reality of Protestantism, which is moving away from “mainstream” churches and towards unaccountable, mediocre “non-denominational” churches. The people running these are often not well-trained (see Joel Osteen, for just one example). These people do not usually treat Scripture “as a work to be approached in a scholarly manner,” at least in my experience, and these people have no problem with sowing even more division (not that there wasn’t enough already).
The average Protestant does not thoroughly research a church’s positions before joining it, but typically just goes toward whichever one that they feel best promotes a sense of “community” (which can mean so many things, and is certainly subjective). This even further lowers accountability and the credibility of the church they attend (I mean, they rarely, if ever, have to defend their ideas), further weakening the Body of Christ. And this is the future of Protestantism.
How is it possible for Christians to be “one,” as Christ requested in John 17:20-21, if we’re not united by a single interpretation (at least on essential matters)?
Of course, Protestants frequently retort that they do agree on “the essentials,” but that just isn’t true. They are very fractured over the necessity of baptism, how the Lord’s Supper should be viewed, “faith alone” (a lot of Protestants don’t really believe in it in the first place, and some Protestant theologians are starting to push a more works-oriented theology), etc. They don’t agree on anything!
Out of curiousity, what exactly makes you think that those Councils had no Scripture to look to?
The books which constitute the Scriptures existed before those Councils ever met to declare an official canon. We're not talking about a bunch of books that suddenly became authoritative after Councils of Bishops met together to declare them as such -- the books were considered authoritative from the moment they were penned and various canons of "inspired books" have existed from the start. These canons were not necessarily uniform but there were books whose authenticity was more or less universally accepted.
Had the Church, after becoming the official religion of the Roman Empire, not held those councils, we would still have Scripture, albeit most likely with differing canons depending where we went. As it were, Christianity did become the official religion of the Empire and the Empire loved uniformity -- it is not surprising that there would be council to decide on a uniform canon to be used throughout its borders. A single canon was decided on based upon a rigorous examination of existing canons. One need not accept the "authority" of the council in order to accept the canon that it set. The other canons faded into obscurity and God often makes use of the accidents of history to implement His will.
I think that your post ignores the reality of Protestantism
For the sake of argument, I could point out various "realities of Catholicism" which are less than kosher even according to Catholic teaching and you would rightly rebute that such has little bearing on whether Catholic teachings, when properly understood, are in fact correct. The Catholic Church is full of Catholics who are not at all unlike the Protestant laypeople you point to. Catholic priests who are not "well-trained" are not entirely foreign to the history of the Catholic Church either. As for the Joel Osteens, if you consider them representative of Protestant ministers as a whole, I would counter that you are missing the larger picture. That would be no different from me calling the former Bishop Weakland representative of American Catholic bishops.
The average Protestant does not thoroughly research a church’s positions before joining it, but typically just goes toward whichever one that they feel best promotes a sense of “community”
Your average Catholic is born into it, which is no different. Conscious converts to Catholicism are rare -- and for the record, I did consciously convert to Catholicism when I was younger. Over the years I came to the conclusion that it is not what it claims to be. Those of us who pick a branch of Christianity based on intensive research and go with it are rare.
How is it possible for Christians to be “one,” as Christ requested in John 17:20-21, if we’re not united by a single interpretation (at least on essential matters)?
The oneness of Christians that Christ requested is one of love. Institutional oneness to the degree that Catholics desire did not even become a possibility until after Christianity became the official religion of the Roman Empire. The Catholic emphasis on oneness in interpretation has less to do with Christ's desire that Christians be one and more to do with the Roman Empire's emphasis on uniformity in religion as a potent political tool. Most ancient heresies were connected to imperial political struggles (some of which were waged against the influence of the Church itself). This was reflected strongly in the struggles between Rome and Constantinople which culminated in the Great Schism. The Protestant Reformation, in turn, played out the way it did in no small part due to political realities of the time. Protestants have learned to lay aside our differences to greater or lesser extents but Catholics and Orthodox have continued to insist on that old ancient imperial uniformity. Insofar as they do, I would argue that it is Catholic and Orthodox attitudes that are perpetuating divisions in Christendom and not Protestants.
Of course, Protestants frequently retort that they do agree on “the essentials,” but that just isn’t true.
I think we all, along with Catholics, Eastern Orthodox, and Oriental Orthodox, can agree on, say, the Athanasian Creed as a good summation of that Faith which is necessary for salvation: Athanasian Creed
All else seems like window dressing to me -- the important part of baptism isn't whether the water actually regenerates or whether it is a symbol of saving faith but rather that we are in fact baptised according to our Lord's command. The Eucharist may become the literal body and blood of Jesus Christ or it might be just a symbolic act done in remembrance of Him -- the important part is that the Lord commanded us to come to His table and to take and eat. Christ's salvation does not depend on whether we believe our acceptance of him was of our own free will or if we believe that it was predestined from before the world was created -- Christ's salvation is entirely dependent upon whether we believe that by dying He destroyed our life and by rising He restored our Life. I don't even think it really matters to Christ whether we believe we are saved by Faith and that our works come as a result of that faith after the fact or whether we believe that we are saved by both faith and good works -- what matters to Him is that we hear his Word and do it.