To clarify, you do get SI, sort of. However, the only thing that can be played during SI is an enhancement that interrupts or negates the card removing your character from battle. Since Ahimelek can't be negated or interrupted, there are no card that fulfill the requirements of a card being played in SI, you simply can't play anything.
Another thing that may cause confusion is the cascade negate. In this example, you are right that negating Ezra's journey would cause Ahimelek to be cascade negated. However, Ahimelek is CBN. CBN applies to bring negated by abilities as well as game rules. If it's CBN, it can't be undone at all.
