Amazing,
There is no justification for using the word "Jehovah" in the NT. Watchtower rationale in its foreword is baseless, because the Apostles rarely quoted an OT scritpure verbatum, but rather paraphrased or used partial phrases.
I respectfully disagree. One of the partial phrases to which you allude is "the Lord's angel" or "the angel of the Lord" which occurs a number of times in the NT (Matthew 1:20, 24; 2:13, 19; 28:2; Luke 1:11; 2:9; Acts 5:19; 8:26; 12:7, 23). Every instance in which this phrase is used in the OT it never once refers to "the Lord" but always to Jehovah so "angel of Jehovah" would have been as much a figure of speech to them as "angel of the Lord" is to those of us brought up on the KJV.
I do think it would have been better to have had 'Jehovah' as a footnote to those texts where it is used rather than in the main text, but the translators of the NWT have explained their justification for including God's name whether you accept it or not.
The translators who replaced "YHWH" in the OT with "Lord" followed an old Hebrew practice used while Israel was still God's people, and thus saw it as a legitimate practice.
The old Hebrew practice to which you refer did not involve changing the text, but of reading aloud 'Lord' or 'God' where the text contained the tetragrammaton. The translators of the English bible changed the text in the same way as the NWT is changed, by substituting one word for another, and if that is acceptable practice in the OT it is a bit hypocritical to condemn it in the NT.
Earnest