Vowels are not used in the "written" Hebrew however they are in the spoken language but unfortunately there is no pronunciation for "J" in the spoken language of Hebrew. As to how the name of God in the Scriptures "YHWH" was spoken orally is a bit of a mystery.
However the JW's must have received some "divine revelation" of the truth somewhere along the way with the countless other false prophets and teachers that have been peddling their so called "truth".
The book of Revelation speaks about a falling away from the Truth in the latter times and so does the Lord Jesus in
Matthew 7:21 Not everyone who says to Me, ‘Lord, Lord,’ will enter the kingdom of heaven, but only he who does the will of My Father in heaven. 22 Many will say to Me on that day, ‘Lord, Lord,did we not prophesy in Your name, and in Your name drive out demons and perform many miracles?’ 23 Then I will tell them plainly, ‘I never knew you; depart from Me, you workers of lawlessness!’…
