Group interventions almost never work out the way they are done on tv. If this is somewhat normal for him then he needs one on one discussion to figure out why he would think that this is ok. If this is abnormal that's definitely something where either a psychiatrist or therapist should be consulted before taking steps. Without knowing everything I would say erratic, impulsive, and inconsistent behaviour should not be trivialized.

I don't ask this lightly: Is it possible your uncle is suffering from serious mental illness? This sounds far more extreme than mere religious conversion or devout religiosity. Is there any history of mental health problems, either in the family generally or with your uncle specifically? If you manage to get through to him at all, maybe some acknowledgement that there's potential harm to himself and to his wife, I'd try to get him to see a psychologist or other mental health professional. I sincerely hope everything turns out all right.

Shit. Immediate intervention. I did it with my sister who was just about to leave her husband on the advice of some JWs - I took a hard line with her and it worked.

He is either being conned or is mentally ill, simple as that, I don't see a middle ground.

crazy people can't be mended