What would JW's/Mormons need to see before they were convinced they did not have the truth and what worked for those who left?

Mark Jones, Ex-Jehovah's Witness of 31 years
Updated Answer on May 31, 2019. Quora.com

Answering as a former Jehovah’s Witness and also as a psychologist…

It’s not just a case of showing them evidence and expecting them to leave. The indoctrinated member needs a reason to leave. And the problem is, the group offers them something in the form of group belonging, a purpose, conditional “friends” and a feeling that they’re privy to some sort of knowledge that’s evaded actual scholars and bible experts.

More and more Jehovah’s Witnesses are becoming aware of the failed prophecies,[1] the pedophilia cover-ups, the child abuse lawsuits,[2] the historical inaccuracy (607BCE)[3] and the 180 turn on things they used to believe[4] - yet they don’t leave. The reason being, if they do they stand to lose more than they’d gain. The indoctrinated member has put all of their chips into the religion. Perhaps all their family are in, all their “friends” are in, they’ve devoted decades to the group. If they leave they lose their family and “friends” and have to face the fact that they’ve wasted many, many years of their lives. They know no one in the real world. Such a thought is too much to face so… hello cognitive dissonance.[5]

Cognitive dissonance will ensure that the person will make excuses for the group and try to justify anything it does. The alternative is leaving, which, as I mentioned, would cause them to lose everything.

However, if they have a reason to leave then they will. If they feel that they stand to gain more leaving than they would staying then they will leave. It could be the group demanding them to disown their own child[6] [7] (which is very common). Or the religion demanding them to let their baby bleed to death[8] (which does happen). Or the group forbidding them to divorce their abusive husband.[9] [10] Suddenly the excuses don’t work anymore and the person see’s the religion’s flaws very clearly and stops making excuses for it.

What worked for me was seeing the glaring difference of Jesus in the bible and the elders and governing body (leaders). Jesus in the bible always came across as approachable, loving and forgiving. I can only remember two elders in my 31 years in the religion who were actually like that. And since leaving it’s come to light that one of those two turned out to be covering up domestic violence in his congregation and disfellowshipped a woman for running away to a women’s shelter for protection. Then I discovered that they were NGO members of the U.N. despite the literature calling the U.N. a tool of Satan.[11] The more I researched the more corruption and lies I saw. Yet, still I didn’t leave. I made excuses. My reason for leaving came when the elders started to interfere into every aspect of my life and no one in the congregation caring when my wife was hospitalized with mental illness. The elders were more concerned with me going out knocking on doors. After being given a reason to leave it was only a matter of time before I did, so I planned my exit. But my previous answers have told this story before.

So in short, in order to leave they need a reason to. Evidence that the religion is false is only half the battle.

Footnotes
[1] Failed date predictions of Jehovah's Witnesses (https://www.jwfacts.com/watchtower/1800s.php)

[2] Uniting church has faced 2,500 reports of child sexual abuse, royal commission hears (https://www.theguardian.com/australia-news/2017/mar/10/uniting-church-has-faced-2500-reports-of-child-sexual-abuse-royal-commission-hears)

[3] Facts about 607 B.C.E. and whether Jesus started ruling in 1914 (https://www.jwfacts.com/watchtower/607-1914.php)
[4] Changed Watchtower Doctrine (https://jwfacts.com/watchtower/changed-watchtower-teachings.php)
[5] Cognitive dissonance - Wikipedia (https://en.wikipedia.org/wiki/Cognitive_dissonance)
[6] https://www.jw.org/en/publicatio... (https://www.jw.org/en/publications/magazines/w20150415/disfellowshipping-a-loving-provision/)
[7] https://www.jw.org/en/publicatio... (https://www.jw.org/en/publications/books/gods-love/disfellowshipped-person/)
[8] Ruling on Jehovah's Witness baby (https://www.bbc.co.uk/news/uk-england-birmingham-26420908)
[9] Shocking Advice for Jehovah's Witnesses on Domestic Violence (
https://www.youtube.com/watch?v=ruUwvITgb48)
[10] The Domestic Violence I Saw With Jehovah’s Witnesses (https://jwvictims.org/the-domestic-violence-i-saw-with-jehovahs-witnesses/)
[11] United Nations NGO Status 1992 (https://www.jwfacts.com/watchtower/united-nations-association.php)