The registrar posted about that question, and I've quoted them here:
(see green). I recommend you phone them to confirm and check, since it is very important that you pay the correct amount by Aug 15th, otherwise you are charged a fee.
You are allowed to alter your payment agreement up to and including September 10, 2007. As long as you have accepted it at least once, you're good. See FAQ here: http://www.mcmaster.ca/bms/student/pdf/SAC_FAQ.pdf
As for your second question, if you have followed all the steps outlined by the Office Of The Registrar (http://registrar.mcmaster.ca /getreg200709/
) then you're 100% good to go. Go through the list and see if you've completed all the steps. After seeing you're 'cleared' and 'fully registered', it looks like you've done everything correctly, but go through the list to double check.
Once again, if you are unsure about anything be sure to phone the Registrar if you need help or have questions. Their contact info is in this thread.