Vidiot:
the first is technically more correct from a grammatical POV
No, it’s not. Would you say “Bob’s Burgers are a tv series” just because ‘burgers’ is a plural? The correct verb is is when referring to a compound proper noun as the name of a singular entity, in this case a religious denomination.
It is only technically correct to say “Jehovah’s Witnesses are…” when referring to a group of JW members rather than the denomination itself.