I don't think you will be able to walk back through EPCOT. The CMs usually will not allow people back into the park after Illuminations. You might try, though (if you don't mind walking against the crowds leaving the International Gateway). You might get lucky. If not, walk back to the BC or BW and choose one of these options:
From the YC/BC back to the Poly you can either take a bus to DD/PI and transfer there to the Poly bus or you can take a cab. A cab will cost about $10-$15, including tip.
If you end up at the BW, you have the same options. If it were me, I'd ask the driver to drop us off at the BW (ours was happy to do that), enjoy the BW activites for awhile and then take a cab back to the Poly.
Enjoy the cruise!