Sorry, I misread your post. My head hurts...
I am using this passage in Luke as support for Sola Scriptura, not as a definition of it. Luke gave me the clear impression that he thought that if one wanted to be SURE about a teaching, then write it down.
That doesn't support Sola Scriptura. At all. It merely means that written material is superior to oral material, as one can readily access all the material in one place. Oral tradition, by nature, is generally not in one person's "head". Even if so, it takes more effort to extract it. Written material is there and available to be read - and re-checked if incorrect. Sola Scriptura, on the other hand, says that anything NOT written is to be discarded. It places written material as the source and arbitrator of any other information. Luke does not at any point make this claim. Nor does the Church. Ever. The Scriptures are vital to the Church - but only when understood in the sense of the Apostolic Tradition, not independently as a work by themselves - which subjects them to MISINTERPRETATION. Even in the second century, the Church Fathers were writing about this very fact and were exasperated! How dare those heretics twist the meaning of our Scriptures! Thus, they saw the need to trace their ancestery back to the Apostles in some of their writings.
Regards
Your last paragraph is well-stated. It certainly reflects something comparable to the practical Orthodox approach to the written Scriptures. They are unquestionably superior to oral tradition because of their fixed nature, unquestioned inspiration, and authority.
The consensus patrum on things not addressed in Scripture or that are perhaps ambiguous in Scripture is, as you say, not an easy thing to determine always (although most of the time it isn't too difficult.) It requires a review of key patristic writings, liturgical texts, iconography, the synaxaria, etc... Where there is universal or nearly universal consensus, then for all practical purposes, this holds the same level of authority as does Holy Scripture.
If the Church appears to have a universal or nearly universal teaching that is in contradiction to Scripture, then it is clearly not the Church speaking. The Arian dominated "church" of an earlier era is an example of this phenomenon. Clearly, an assembly of bishops voting a certain way does not have an authority that is above the clear witness of Scripture.
On forums like this, we tend to focus on "trigger-points" of theology -- hot topics that more often than not are addressed obliquely or seemingly ambiguously in the Scriptures. In those instances, the Orthodox approach is obviously to lean heavily on the consensus patrum. The consensus patrum does not mean that all sources must agree -- far from it. It is a consensus, and therefore some writings of Fathers will be outside the consensus and do not need to be taken into consideration (liturgical texts as a general rule are practically never outside the consensus, because of the nature of their origins and transmission -- and because of their poetic nature, they are generally less problematic in general.)
By contrast, what Scriptural evidence that does exist must *always* be accounted for, and must *never* contradict the consensus. Protestants understandably view our readings of Scripture as being sometimes forced or stretched, and there is little that one can do to counter that opinion, given that we each start from different sets of assumptions.
But I think that it is a useful corrective to point out that for the vast majority of things, the Scripture is what is clear and what we in the post-Apostolic era have always shaped our thinking to fit.
The Scriptures are the matrix of the patristic writings, not the other way around. When one reads the Fathers themselves, their deep reverence for and deference to the Scriptures is as clear as the summer sun.
I write this because often we deal with caracatures of our views on Scripture, and because we sometimes, in the process of dealing with these more difficult issues, contribute to those misapprehensions. We have a duty to try not to make such contributions, and we all have the duty to try not to mischaracterize each others positions. A basic principle of debate is that one be able to state the other's position in a way that they would agree with it. Then, discussion and debate can proceed with a relative lack of rancor.
Sola Scriptura does not say to throw out anything not written. It says that God's written word is the only authority. Extra-scriptural things can be fine if they do not "offend" scripture. I believe that the opening to Luke is in support of this by taking the step of showing that something must be written in order to be sure. That's authority over all that which is not written. (Of course what is written must be God's word, and in this case, it was.)
As I have already acknowledged, I do not claim that the Luke passage is THE slam dunk for Sola Scriptura, but is useful in general support of it. We have also seen other scripture.
And you are right that Sola Scriptura does place God's written word as the source and arbitrator of all other information. Therefore, everything else must be interpreted through the Bible, rather than the Bible being interpreted through everything else.