I want to knock one idea on the head. You CAN book a round trip on the day that booking opens for your outward. You do NOT have to wait until the return date arrives.
Think about it. On UK to Australia routes fares have a 1 year validity but require advance purchase. That means you can go London to Sydney on 27th December 2010 (for which booking is already open) and return 26th December 2011 (for which booking is not yet open). What do the airlines do? They make your outward booking and then put a note and a time limit in the booking for the return flight. When the time limit expires (it gets set for the date bookings open) they make the booking and confirm it to you. I've never done it with BA (I know others who have) but I have done it with Air NZ (indeed mine was even more complex - I made a LHR/HKG booking the day they opened) and then wanted HKG/AKL 4 days later, AKL/SFO two months after arrival in AKL and LAX/LHR 11 days after arrival in SFO. The lad who did it for me had to go and ask but it worked a treat.
As for the cheapest days you need to think when people will want to travel and then pick a different day. Not only when people from your starting point but from change over points and people going home to the destination. You can't factor in things like private parties but school holidays, public holidays, religous festivals etc you can research and take account of. The days people do NOT want to travel are most likely the cheaper ones (e.g Friday 13th). Easy for me to explain, difficult for you to implement.