Hi David,
I e-mailed Disney about purchasing APs in advance with DC discount and was told that it isn't possible - they must be purchased at the gate to get the discount. However, Penny (Disneymad) posted that it is possible to buy in advance (although not online), but I can't remember exactly how. It certainly isn't unusual for Disney fans to know more than the CMs - I guess it's just luck whether or not the one you get knows the answer to your particular question.
My understanding of AP exchange certificates is that they run from the date of exchange and that's certainly always been our experience. We bought our most recent exchange certificates online back in August time last year and activated them in December - they run until December 2002. I think it's the renewal policy that's changed. You used to be able to renew your APs immediately on expiry, but not activate them until your next visit. Now they run for 12 months from date of renewal. In practice this isn't an issue - you just let them expire and buy new ones. Of course you don't get the renewal discount.