Yes, onward passengers are deplaned and all belongings must be removed and taken with you as if ICN was your final destination...except for your checked in baggage of course. It is only a short 1hr transit at ICN.

Yes you do have access to the star lounge at ICN, although if my memory serves me correctly I don't believe they have any showers there. Then again I've never quite have payed attention to that as I'm usually in and out if the lounge within 40min.