You can walk over to BoardWalk from Epcot. You exit from the International Gateway (near United Kingdom) and walk takes less than 10 minutes leisurely pace (there is also a boat but that actually takes longer). As to parking, BoardWalk has two lots. The main lot has a guard gate and you cannot enter unless you are actually going to BW rather than Epcot although they tend to be a little more relaxed on that if you don't show up until evening as they assume you will be going to the BW then for evening events. The overflow lot across the street from the main lot is open to anyone. Thus, if you are going to Illuminations and then to ESPN, you might want to try that. Getting back to PO by bus is a pain in the butt. If MGM is still open, best is to take boat or walk over to MGM and catch your bus there. If it is not, then you will need to take bus to DD and then catch PO bus there. Other option if you don't take car is to get a cab from BW back to PO; lots of cabs are available there at night.