As far as how to distribute the hashrate from completed accounts, I don't know enough about how many accounts have how much in them to say what would be the best. Although another idea would be to randomly pay out to one account at a time (0.01 BTC) given to a random account at a time with some extra weight to larger accounts. Or split things between different plans 75% to n oldest accounts, 20% randomly, 5% to largest account). I'll be happy if I can get paid out in the foreseeable future.

As for favoring new vs old deposits, that's something that has to be done. If old accounts are favored there's no reason to create a new account, keeping some of the completed account hashing power in the new deposits even makes sense; faster returns is more incentive to add money into the system. If there's no new money there's no new hardware, this is something that should probably have happened for each successive generation of hardware. Getting MHs when you have to pay for GHs doesn't make a lot of sense and just seems like tossing more money into the same pit that took your money in the first place. People have to be able to profit off their own money and then everyone else can recoup "lost" money.