You are correct YHWH is in the MT of Psalm 102 I made a mistake about that. (Edit: No I checked again - I think I was right the first time, YHWH is not used in this verse in the MT.)
But on the wider point what I am saying is that Kyrios in the LXX didn’t always translate YHWH. Sometimes it translated Adonai as it does in its second occurrence in Psalm 110.1.
What did the original LXX of Psalm 101.26 read? If it was understood as a Messianic Psalm in the LXX then it’s possible that the “Lord” here translated Adonai rather than YHWH because the Davidic ruler was viewed as Adonai/Kyrios. The Messiah was not viewed as YHWH but as his servant, as the ubiquitous use of Psalm 110.1 in the NT demonstrates.
This isn’t an argument Watchtower has ever made as far as I know. It is me trying to work out what is going on in Hebrews 1 and the Psalm it quotes.
What convinces me that the early Christians viewed Jesus as distinct and subordinate to YHWH is that when applying the title “Lord” to Jesus they specifically chose Psalm 110.1 where the “Lord” is distinct and subordinate to YHWH. That’s a rather odd choice of what they were really trying to say is that Jesus = Jehovah, don’t you think?
Psalm 110.1
Jehovah saith unto my Lord, Sit thou at my right hand,
Until I make thine enemies thy footstool.2 Jehovah will send forth the rod of thy strength out of Zion:
Rule thou in the midst of thine enemies.
Jesus, as “Lord”, is distinguished from and is subordinate to Jehovah/YHWH in this Psalm, and everywhere it is quoted in the NT, such as Acts 2 and elsewhere.
As far as I can work out YHWH is always distinguished from Jesus in NT quotations of the OT. This is true in the vast majority of cases even Trinitarians would agree. There are a handful of cases such as Romans 10.13 and 1 Cor 2.16 where Trinitarians would argue YHWH = Jesus. (In particular the scholar David Capes makes this argument.) But even in those handful of cases it still makes sense to read the text as referring to God rather than Jesus. For example in Romans 10.13 people call of the name of Jehovah because it says God raised Jesus from the dead in verse 9. So an acclamation to God fits here too. In 1 Cor 2.16 Paul’s argument seems to be that scripture says we can’t know the mind of YHWH but that Christians can know ‘the mind of Christ’, who perfectly reflects God’s character. So again a careful distinction is being made between YHWH and Jesus that is lost if you fail to note the difference between YHWH and Jesus as ‘Lord’.