Greetings,
First, this question is logically flawed. It operates under the false assumption that there are NT manuscripts which are unchanged. Any belief in a totally unchanged Bible text goes against ALL the evidence.
Only some "KJV-only" people believe that God preserved the Bible word-for-word, and they cannot logically maintain this position since even the KJV differs from its Hebrew and Greek text. EVERY Bible translation in existence must choose between various text readings and make a determination as to what word(s) were lost or added. When this is done using sound textual principles we have a very accurate text and versions which are generally in agreement.
While God did not preserve the copyists from making any mistakes, He did limit the degree of variation so that the *doctrinal* content of the NT was not affected (except for several well known spurious verses added by Trinitarians in the 10th cent.).
The NWT inserts the Name Jehovah in the NT because textual and translation principles demand it. Many other translators have acknowledged these principles and placed Jehovah "Jehovah" in the NT, and most other translators have acknowledged these principles because they have capitalized LORD in the NT which ALWAYS denotes Jehovah (Mt.22:44; Ac.2:34).
First, a translator's prime concern is with transmitting the meaning of the original writer. So if the original writer directly quoted from the O.T. the absolute semantic equivalence of KURIOS in the target language must be "Jehovah" (e.g.; Mk.12:25-36; Ac.2:21, 33-34; Rm.10:13) (see Girdlestone's Synonyms of the OT; 43).
"In the NT, likewise, KURIOS, when used as a name of God...most usually corresponds to hwhy Jehovah, and in this sense is applied." --A Greek and English Lexicon to the NT, by J. Parkhurst
Second,, there are the occurrences of the tautological "Lord God" (KURIOS hO THEOS) which provide strong evidence indicating the original N.T. text used the divine name. This is proved by the LXX where "Lord God" everywhere signals where "Jehovah God" was replaced. (Mt.4:7,10; Mk.12:29,30; Lk.1:16,32,68; Ac 2:39; Rev.4:8; 11:17; 15:3; 16:7 &etc.).
Another reason would be if the context demanded that the word KURIOS be understood to mean "Jehovah." A good example of this is 1Cor. 2:16. We would have a nonsensical meaning or an outright contradiction if we did not somehow identify Jehovah as the "Lord" who's mind we don't know as contrasted with the Lord who's mind we do have. (Cf: Mk.11:9; 12:35-36; Ac.2:34; 3:19,20; 4:26,29,30; 2Tim.1:18).
In these places "Lord" is textually accurate, though factually incorrect. Recognizing this, several versions make use of capitalization in Mat.22:44, Mk.12:36, and Ac.2:34: "LORD"!--KJV,JPS.
Such principles cover all the places where the NWT and others have restored Jehovah to the text of the N.T.
Next, other scholars besides Witnesses have recognized the evidence that the extant Greek N.T. manuscripts were altered and the Divine Name was removed in the second or third centuries.
First, it is a fact that the Divine Name Jehovah appeared in the Greek translation of the O.T. (LXX) through the first century C.E. This proves without a doubt that copyists of the 2nd century replaced the Divine Name in the LXX with "KURIOS." By logical deduction, and the evidence outlined above, we can safely conclude the same thing occurred for the N.T. texts.
Professor George Howard observed: "The writers of the New Testament included without doubt the Tetragrammaton in their quotations".--Biblical Archeology Review, 3/1978, p.14.
Notice the further quotes from George Howard's "The Tetragram And The New Testament," Journal of Biblical Literature 96/1 (1977) 63-83:
"Toward the end of the first century Gentile Christians, lacking a motive for retaining the Hebrew name for God, substituted the words [kyrios] and [theos] for the Tetragram....Thus somewhere around the beginning of the second century the use of surrogates must have crowded out the Tetragram in both Testaments. Before long the divine name was lost to the Gentile church altogether except insofar as it was reflected in the contracted surrogates."
For the Christian, the most important evidence is the evidence from Scripture:
God made it clear that Jehovah would be his Eternal Name (Ex. 3:15, Mic. 4:5; Jer.23:27).
Christ specifically stated that his determination was to make Jehovah's name known to Christians (Jn. 17:6, 26. Not inserting that NAME in the NT would be a failure to follow Christ–a denial of being Christian.
All these facts give corroborating evidence to every reasonable person that Jehovah must have been included in the original N.T. books.
Leaving the most holy name unrestored in the N.T. would be falling into the purpose of those who hate that Name. These are the same ones who have removed the divine name from the O.T., thereby receiving God's condemnation (Rev.21:19).
Yours,
BAR-ANERGES