In the old testament you have an invisible god named Yahweh but you have divine manifestations of Yahweh ( the angel of the lord, word of the lord, wisdom of the lord etc etc ). It is not entirely clear who is what and what is who.
The new testament is the authors realizing that this manifestation was Jesus. The ambiguous use of the tetragrammation in the new testament is by design, not corruption.
Calling the tetragrammation Jehovah and adding it to the new testament is trying to clear up the ambiguity and it is just not correct. It is a mess and a mess by design.