You can use the "Diplomat/Crew" security line if you do your check-in and immigration at Seoul Station OR City Air Terminal at COEX in Gangnam. You are required to check in 3 hours before your flight (at the latest). At these places, they will issue boarding passes, take your bags and you will go through immigration there. Then, from Seoul Station you catch a train (COEX uses limousine bus).

If you do get to ICN and find that the immigration lines are a mess, you are allowed to use the crew/diplomat line only at the boarding time listed on your boarding pass. Just find someone working the line to help you. When it's the time on your pass, you can cut through and use the crew/diplomat lines. There's still enough time to make it to your flight.

As far as it being a "zoo," I typically wouldn't say so. For almost five years that I've lived here, I've only seen it a "Zoo" once. Two weeks before Christmas on a Saturday morning. I waited 45 minutes in the Web Check In Line (which actually didn't even work when I tried on my phone). Other times, I always arrive an hour and a half or so (sometimes less) before my flight and have no problems. The "zoo" depends on which airline, flight and time of year. Don't worry too much about it!