Dinner at WCC usually starts around 5:00 pm, so there isn't much of an earlier time you can get. As others have said you have to catch the boat from WL to MK and that is about a 15 minute ride, and depending on how long you have to wait it might take a while to get to MK.
The only thing going in your favor is that I think the parade runs from Frontierland to Main Street, so you might have a few minutes leeway (from when it starts to when it reaches Main Street).
I really doubt you'd be able to get through dinner, catch the boat and make it into the MK in time for the parade, without feeling really rushed. You'd have to eat really fast (if you try, probably the best option is to only have an entree - skip the appetizer and dessert or order the dessert at the same time as the entree and have it "to go").
(For reference in December, we had a 5:05 pm dinner reservation at WCC and made it back into MK at 7:30 pm.)