The basic contrast is between in, which does not present occurrence as assured, and idha, which normally does. Their rhetorical difference exceeds the simple distinction of “if” and “when.”
When in occurs with a known or impossible premise, possibilities include feigned ignorance, strategic concession, rebuke, counterfactual reasoning and adaptation to the hearer. The speaker’s true knowledge, the hypothetical frame, and the state of diverse addressees must not be conflated.
The final discussion of addressing mixed groups leads into the next independent topic, taghlib, treating one expression or category as dominant in a combined reference.