My first choice is to go in the morning, come back for a swim, then go back in the park. THat being said, I think it is quite possible to do what you are suggesting. Depending on what you are interested in seeing. If you have been in the past and are not going commando to see everything, I think you could have an enjoyable time doing it this way.
I would look at each park you plan to visit differently.
In EPCOT, if I were to go in late, I would start in the front by late afternoon , I think most people are out of the front of Futureworld. I would be cautious and realize however, while you might get to see attractions, fast passes might likely be gone for the day for the big rides when you get into the park. If not, go right to the must do fast pass attractions to get fast passes.
In MK, I would work from Adventureland to Tomorrowland in a circle around the castle. I think this would work because most people I believe start in Frontierland/Adventureland to go on Spash and Thunder first, hopefully, by the time you got there, people would have cleared out of the area.
You might want to figure out how important seeingthe parades and evening shows are? You can ride a lot of popular rides during parades and fireworks
