most "RANK AND FILE" JW's are mentally unstable
Mental illness is a medical condition, diagnosed by professionals using specific medical criteria.
Believing in something that isn't true doesn't by itself mean someone is mentally ill. If that were true, most kids would be mentally ill because they believe in Santa Claus. Or people who are superstitious about the number 13. etc.
I don't doubt mental illness is common among dubs, as it's common among the general population and dubs have additional risk factors. But I'm having trouble with the word "most" in the OP, in absence of evidence.