The most likely scenario is the ex-husband. The mob girl/abortion/hollywood shot caller/party girl angles are interesting but I really suspect those were floated and played up by Benner's attorney for custody purposes.
In reading through the LA Times coverage of the 1950 custody hearings, Benning (through his lawyer's actions) came off to me as ruthless, cold and vindictive - even by un-PC 1950's standards. He seemed hell-bent on dragging Jean's reputation through the mud. Keep in mind that his ex-wife had already been missing for a while. Benning was fighting against Jean's mom, his ex-mother-in-law who just wanted some visitation. Benning's lawyer was pointedly grilling Jean's mom about Jean's whereabouts as if she had just run away on a weekend bender. Benning also complained that grandma kept bringing up Christine's "real mother" during her visitation.Grandma not wanting her granddaughter to forget about Jean was apparently an unpardonable slap in the face to Dexter and his new wife. Finally, in the face of having to deal with a limited amount of court appointed visitation, Benning leaves California for Florida, never complying with the court, hiding in plain sight as best as I can tell. Jean's poor family kind of drops it and so Benning scores a defacto "win" in the custody case.
For decades, he lives a new life in Florida, never speaking of his missing ex-wife despite being approached for comment on a regular basis, I'm sure. Is that really lingering bitterness over their 3 year marriage/50 year old custody case or something else?
When E Television tracks him down, he has no comment and also says Christine has no comment. He has since passed away but his wife, kids and grand kids are all easily find-able. Neither Christine nor anyone else has ever commented on her famous biological mom, that I'm aware of. Even in death, no one dares entertain the idea that Dexter did anything but rescue Christine from a toxic situation.
It all paints the picture of someone who had a dominating personality, control issues and didn't like to lose. He wanted to start a new family with a new wife. The only thorn in his side was Jean. She didn't let him have Christine and had the gall to ask for more child support at a time when he just wanted to be done with her. PTSD may also be a factor in that he recently returned from WW2 combat. Was Dexter a support person in the armed forces? No, a pilot, of course. He liked to be in control.
He did not become a hermit in Florida. He was living his life in the open. Dexter went on to run his business and have a social life in Florida. I'd be curious to hear from anyone who interacted him after 1950 - suppliers, customers, guys from the yacht club, neighbors, fishing/golfing buddies, Kiwanis Club associates, etc. Was he an introverted good guy or an alpha male hard-*advertiser censored* who you crossed at your own risk? In 2001 Jean's sister said she was still scared of him and not comfortable speaking to E on camera.
My opinion is that he snapped under the pressures he was facing in California, went AWOL, got the family life he really wanted in Florida and went on to lead relatively normal life.