SBF et al rely on George Howard's study to say that the "original" LXX and the "original" NT may have had the Tetragrammaton... but Howard's study at bestallows for the possibility of the Tetragrammaton being in quotes of the OT...
That’s incorrect. Howard’s original article suggested that the divine name was used, not only in quotes, but also in certain established OT phrases, such as “angel of Jehovah”, and “word of Jehovah” you have been discussing here. It’s worth bearing in mind that the NWT was published before Howard’s article and has never been bound by his methodology, but in the matter of using the divine name in the phrase “word of Jehovah”, he supported the NWT’s approach.
I quote the relevant part from Howard’s article here:
If the Tetragram was used in the NT, how extensively was it used? Was it confined to OT quotations and OT paraphrastic allusions, or was it used in traditional phrases, such as "the word of God / Lord" (see the variants in Acts 6:7; 8:25; 12:24; 13:5; 13:44, 48; 14:25; 16:6, 32), "in the day of the Lord" (cf. variants in 1 Cor 5:5), "through the will of God" (cf. variants in Rom 15:32)? Was it also used in OT-like narratives such as we have in the first two chapters of Luke?
We have discussed Heb 1.10 elsewhere. This quotation is from the Greek version of the Psalm which may have been interpreted as a messianic Psalm and used kyrios for the Hebrew equivalent Adonai rather than the divine name, just as was the case with the second “Lord” in the often quoted 110th Psalm.
Btw can any Trinitarian explain why Jesus as “Lord” adoni/kyrios is distinguished from and subordinate to YHWH in Psalm 110.1? Judging by the popularity of that Psalm among early Christians they were in absolutely no doubt that Jesus was distinct and subordinate to Jehovah.
Jehovah declared to my Lord:“Sit at my right handUntil I place your enemies as a stool for your feet.”
You are completely correct that the NWT may be not be right about every instance where the divine name should be restored. That’s an easy point to concede because they have implied as much themselves and have simply claimed to do the best they could on the available evidence.