I've had lots of trouble trying to use the website - it's unreliable and not at all user friendly, if you ask me. I no longer trust it.
It's actually a lot easier to just call MS and have them do it. The results (and the $3 handling/mail fee) are the same - you get a voucher in the mail (unless you ask for the will call option).
Yes, you can renew a AP or PAP (for which you paid full price) and pay the DVC-discounted renewal price.
Best wishes -