NeonMadman....I agree with you about the lack of evidence that YHWH was ever in manuscripts of the NT (such that the NWT's "restoration" of the divine name is really an act of eisegesis); however I do disagree with your statement that
The only way that alterations could be made that would be undetectable through textual criticism would be if the actual autograph copy written by an apostle or other writer were somehow altered on the way to its destination.
Textual criticism has little ability to detact early alterations to the text because there are few manuscripts available that go back to the second century and those that do (papyri) are very fragmentary and of limited use compared to the codices of the fourth century. The vast bulk of manuscripts (80-90%) attest the inferior Byzantine or Majority text-type which is rife with scribal errors, interpolations, and omissions, whereas the earlier papyri of third and second centuries, while attesting a superior Alexandrine text-type, are fragmentary, contain their own errors, and are still at least a century removed from the originals. The papyrus that comes to closest in time to the original, the Rylands fragment of John, is a tiny scrap attesting just parts of two or three verses. The earliest manuscripts for 1 and 2 Timothy date only to the fourth century. Since the archetypes of the major textual families date back to the second and third centuries, it is certainly possible that early textual changes can remain undetected by textual criticism. In other cases, it may be unclear which option is original because both variants have early attestation. That the second century was a time of textual fluidity for the NT can be seen in the patristic remarks about heretics like Marcion (c. 145) altering the text of Luke and Paul's epistles, the popularity of gospel harmonies (c. 150 for Justin Martyr's harmony), and the patristic citations of the NT which themselves evidence features of the later text-types.
There are also many ways in which changes may have crept into the text of the NT. First of all, what was actually the original autograph? It is generally thought that Paul's scribe made a copy of outgoing letters for Paul's own use; thus minor discrepencies may arise between the two original versions of the letter, and the polished letter sent to the church may not be exactly the same as the copy retained by the author. Moreover, some of Paul's letters were circulated between churches (cf. 1 Thessalonians 5:27; Colossians 3:16), leading to the production of local copies. The publication of Paul's epistles as an assembled corpus, however, likely did not occur until after Paul's death and went through stages: (1) An initial seven-letter corpus of letters to the churches, later imitated by John the Presbyter in Revelation (c. 95) and Ignatius of Antioch (c. 115), (2) the addition of 2 Corinthians (which was unknown to Clement of Rome in his letter to the church at Corinth, c. 95) and private notes like Philemon to the corpus, and (3) the addition of the Pastorals. The Pauline editor(s) responsible for the initial publication of Paul's letters, perhaps disciples of his like Timothy or Silvanus, would have thus had opportunity to add their own improvements to strengthen or clarify his points, and even Paul himself would have had the opportunity to revise his own version of his letters. 2 Corinthians is also generally believed to be an edited version of several shorter letters to the church, and the editing process would have also allowed subtle changes to enter into the text. It is also probable that, in the climate of proto-orthodox and proto-gnostic disputes in the first half of the second century, different schismatic groups that both appealed to Paul as their founder produced differing versions of the letters that were influenced by these debates (so that by c. 150 different recensions of Paul's letters were in circulation, including the proto-New Testament assembled by Marcion).
In the case of the gospels, it is known that they went through several different editions. Since they originally circulated in rather provincial contexts (e.g. Matthew most likely originating in a non-Pauline Jewish-Christian community in Syria), the early versions may not have been widely circulated and quickly superceded by the published editions. This is most clear in the case of Mark (c. 70), which was utilized as a primary source in the gospels of Matthew and Luke (c. 80-100) -- such that 90% of the Markan text is reproduced in Matthew and 60% in Luke. By comparing common omissions and common additions against the canonical Markan text, it is possible to detect an earlier version of the text used by the first and third evangelists which no longer exists as such in Mark. These divergences from canonical Mark are systematic in that the additions and changes to canonical Mark contain distinctive vocabulary and contribute to a baptismal/resurrection mysticism (cf. especially Mark 4:11, 8:31, 9:15, 25-29, 10:24, 38-40, 14:51-52, 16:5), which point to a redaction of Mark after its use by the authors of Matthew and Luke (cf. also the Bethesda section of Mark 6:45-8:26, missing in Luke). There is also the famous lacuna in Mark 10:46 (filled in by Secret Mark), which indicates that something in the pre-canonical version of Mark was omitted here. The original ending to Mark is also likely missing (16:8 seemingly ends in mid-sentence and the Galilee appearance promised in 14:28, 16:7 never occurs), leading second and third century editors to invent several different endings to the gospel; one possible explanation is that the Galilee appearance was present in the original text of Mark (thus Matthew 28:16-20 has such an resurrection appearance), but the editor who devised a more mystical version of the text intentionally omitted the appearance to imply that promised Galilee appearance lies at the beginning of the book in the Galilee ministry that follows Jesus' baptism (thus completing the baptism=death/resurrection symbolism). As for Luke-Acts, there is the problem of the remarkable divergence between the Western and Alexandrine/Majority text-types (cf. especially Luke 3:22, 6:5, 22:17-20, Acts 5:15, 8:36-39, 13:8, 15:20, 29, 21:25, etc.), both deriving from second-century archetypes, and indicating that two major editions of Luke-Acts circulated in the second century (in addition to a third, the version of Luke by Marcion). Although the Alexandrine readings are usually preferred as superior, some of the Western text variants were attested first by the mid-second century (cf. Justin, Dialogue 88, 103.6). The gospel of John also a number of major textual problems and accretions, as discussed by Bultmann, Koester, and other scholars.