This focus on "Jehovah" results from the thought bubble in the 1930s by the Judge, Mr Rutherford, when he invented the title "Jehovah's Witnesses".
In doing so, Rutherford mistakenly used the word invented a few centuries previously by the Roman Catholic Church, so that not one JW calls on, uses or knows anything but this concocted word: Jehovah.
The context of "you are my witnesses says YHWH" at Isaiah 43:10, shows that it is being used derogatively: the people had witnessed (seen) the power of YHWH yet they had chosen to forsake him, and that was the reason for their parlous state, the captives of Babylon.
(As a footnote to this passage at Isaiah 43, note that this was written during the Babylonian Captivity, some 200 years after Isaiah lived and died.)
Doug