Hi there, Heather!
Your best bet, rather than parking at Epcot that day, is to park at the centrally-located Transportation and Ticket Center (TTC).
From the TTC, you can hop a monorail to Epcot; while this does add fifteen minutes or so on to your travel time, this minimal bit of front loading will ensure that you won't have any issues with monorail or bus service directly back to Epcot following the fun of the
Pirates and Pals Fireworks Voyage.
Give a quick check to the
Parks Hours Calendar to confirm that the Magic Kingdom will be open later than Epcot on the night of your trip, and please remember that monorails do typically continue running for a while after a Park closes (but yes, likely direct transport to Epcot will be finished following Pirates and Pals).
Have a wonderful time!
See you later, celebrator,
-Deborah B.