never buy the epp in advance unless it is a holiday period.
if you are only standing in line for 15 minutes per attraction, the epp is not worth it.
the epp can be bought inside the park at anytime of the day in case you find the lines to be rather time consuming.
you buy the epp based on park and date.
like the pp stated, no refunds.
if you go on a day that it rains, the crowds will lessen.
not many stay in the parks when it rains heavily.
you will not have lines then.