i suggest you wait until you are in the park to make the decision of buying the epp.
if you arrive before the gates open, you will be able to do many things before the crowds begin.
the epp is only valid for one ride/attraction each.
the passes are non refundable.
once you buy them and if you don't really need them, they are yours still.
they are valid only for the park you designated them for.
if you want to do both parks, it is an additional cost but cheaper than buying them separate.
the cost of the epp is an indicator of predicted crowd level for that day.