If one holds to a view of "Sola Scriptura" that "only what is expressly mentioned in the Bible can be believed", then yes, that would render "Sola Scriptura" self-refuting. But that isn't the historic way Sola Scriptura was understood, or more accurately, the way Sola Scriptura was done--as was said earlier, it is more historically a method rather than a "doctrine" per se.
I must say that I find your attempt to distinguish and salvage Lutheranism wholly unconvincing. I have recently been listening to Jordan Cooper and his view seems similarly incoherent, although he is generally a careful thinker. I don't deny that there are different interpretations of
Sola Scriptura among Protestants, but none of the views that I have seen come anywhere near adequately addressing the argument of the OP. It seems to me that this is a symptom of the lack of philosophical training among contemporary Protestants, but this leaves me wondering if the early scholastic Protestants were making the same errors. The most honest rejoinders I have seen are nothing more than
tu quoque, which I suppose is better than nothing.
Regarding your post:
Sola Scriptura, therefore, is not understood to mean, "only what the Bible says can be believed", but rather following the Normative Principle, that which affirms and is in agreement with Scripture is also to be believed. If Scripture does not command nor prohibit something, and if it does not disrupt the peace and health of the Church, then it is permissable.
But you clearly do not believe this, and the canon is the easiest way to see this. Someone who adds to the canon of Scripture can easily do so while remaining within the parameters you set out, and yet you would nevertheless reject their addition on the basis of "
Sola Scriptura," which proves that your definition of
Sola Scriptura is mistaken.
There may be some problems that
Nuda Scriptura succumbs to which
Sola Scriptura does not, but the OP is not one of those problems. When it comes to the OP that is a distinction without a difference.
In Lutheranism we speak of the Norma Normans (The Ruling Rule) and the Norma Normata (The Ruled Rule)...
The technical problem at hand is that that which defines the boundaries of an authoritative document must itself be as authoritative as the document, if it is to be 'dogmatic'. If the boundaries of Scripture, such as the canon, are only a
norma normata, then they cannot be dogmatic. And since the boundaries are not included in Scripture itself, they must not be dogmatic (on Protestantism). So if the Protestant wishes to be consistent then he must affirm that
Sola Scriptura is a fallible doctrine which is not on par with Scriptural doctrines. One reason Protestants won't do this, especially as a matter of praxis, is because it throws doubt on the boundaries of Scripture, and this undermines Scripture itself since a document without boundaries is not identifiable, and is therefore impotent.
I drives me a little bit bonkers to watch Lutherans--and other Protestants too, but Lutherans like Cooper should know better--attempting to continually evade the central problem here. Granted, this happens in all traditions, but it more often happens on the doctrinal
peripheries.