Your dealer is smoking something if they claim you need to drain coolant to replace the spark plugs. They are just ordinary spark plugs, and you change them the same way you change them in any I4; should take 10 minutes, even with the most ham-handed and green dealer tech.
And the car does not have a defined maintenance schedule for the spark plugs. It's whenever MM Code 4 appears. (The same service is also a valve clearance check, but that shouldn't require a coolant drain either, although it would take longer and not be stupidly-trivial, just easy.) FYI, the coolant itself is MM Code 5.