Personally I think putting passengers who use online check-in in boarding group D is the best option, considering that these people will only have to arrive at the airport about 15 minutes before STD. Therefore, passengers who do not use OLCI still have an incentive to arrive at the airport nice and early so that they get the best choice of seats, whilst the incentive for OLCI passengers is they get to arrive at the airport as late as possible, almost like strolling onto a bus.