Does imagining naked bodies and intercourse with them expel one from the fold of Islam and necessitate Ghusl and uttering the Shahadatain? And what are the implications of involuntary sexual dreams and fantasies? And does having previously touched the urethra necessitate anything other than repentance? And is Ghusl obligatory for a sexual climax without ejaculation?

What you read, that these fantasies are disbelief, is false. These fantasies are not disbelief, and while they are forbidden, they are a type of sin that necessitates repentance. You are not sinful if you see these things in a dream or between sleep and wakefulness, or if they overcome you without your summoning them or intentionally indulging in them. You are not required to do anything by placing your hand on your private parts, and if a sexual tremor occurs and you do not see semen, then ghusl is not obligatory upon you.