Before you stumble over the principles of Lutheran textual criticism and hermeneutics again, you should read Robert Preus, “The Theology of Post-Reformation Lutheranism” (St Louis: CPH, 1970). I believe it is still in print. Dr. Preus’ dissertation at Edinburgh was on this topic, so he should know–and no one questions his orthodoxy that I know of.
Re. textual variants, Preus writes; “After Christ there were indeed errors made by copyists out of ignorance or carelessness. But there was no general corruption of the Scriptures. A comparison of the extant manuscripts with all the variant readings bears out this fact. The variant readings are generally of a technical nature and of little importance, such as omissions, transpositions, spellings, and the like, and can be easily corrected. . . . The extant apographa are today authentic becuase they contain not merely the content but the very words of the original Scriptures. A copy of a document is as valid and normative as the original.” (ibid., p. 306). This was the view of the Lutheran orthodox fathers which Preus agrees with.
If you read what Preus says, you will say that the variant readings are GENERALLy of the nature described. But there are also a few SPECIFIC cases, which are often the object of interest and are more significant (like the ending of Mark).
Textual criticism is an ongoing area of scholarship, because copies of the texts continue to be found through archeological and archival discoveries (e.g., the Dead Sea Scrolls). Lutherans do not have a pope to give divine sanction to a particular text or translation (e.g., the Vulgate), nor do we give that right to our kings (e.g., the King James Version). Texts are historical objects and require the proper use of historical principles (i.e., Luther’s historical-grammatical hermeneutic). Therefore the study of the texts will continue unabated until Jesus returns.
Martin R. Noland