Also, since it says "no multi-person reservations", do l have to call and see about getting confirmation numbers for everyone?
Yes, call customer service and get the reservation split into separate reservation numbers so you can do the internet check-in. I hope eventually SWA will modify their system so multi-passenger reservations won't need to be split and can do internet check-in.
If I print out the boarding pass after midnight, does that prevent us from having to be at the airport and stand in line for hours?
It depends. One of the reasons for Southwest's success, and continued profitability, is that they get more use out of their airplanes and gates every day than most of your traditional carriers. Depending on your airport, there may be a another flight leaving your gate 30 minutes before your flight, if that's the case, you really can't get in line until the earlier flight boards. Or there could be an hour (or more) between flights at a gate. In that situation, I'd ask the CS agent if your flight is originating (i.e. will be empty when you board) at your airport, and if not, how many "through passengers" are on board. If the flight is originating, I wouldn't stress about being at the front of the "A" line. If the flight isn't originating and there are a significant number of through passengers, I'd stake a claim to the front of the "A" line as soon as you can. (Southwest's 737s seat between 122 and 137 passengers, so you can compare through passengers to the number of seats available.)
If you must have two seats in the front (i.e. first 8-10 rows), I'd plan on getting in the "A" line as soon as possible, to avoid the stress of worrying about it you'll get those seats. More than likely you'll get two seats together unless you are at the back of the "C" line, the just may not be in the front.
Its easy for me to say because I really don't mind where I sit (front or back, although I do prefer the front), but don't stress about it, and enjoy your trip!