We have never done FP for Illuminations, so I can’t speak to that question. I don’t know if you could count on getting it as 4th FP though. I would think that with a 7:10 ADR, you would have time to get to whatever location you choose for Illuminations.
Yes, there will be a mass exodus of people. If you mosey just a bit, you might find it is easier to navigate – or just don’t leap up and start moving as soon as the show is over. It actually isn’t too bad walking out of the park in the exodus – EPCOT has such wide walkways – what is awful are the lines for the buses. If you want to avoid that, do as the PP mentioned and shop a bit, take some pictures, soak up the night time ambiance of EPCOT for a bit. You’ll still be spending time, but at least it won’t be standing in a bus line. Or, you could drive and hot foot it out with the crowd. You would certainly make it to your vehicle before all the bus riders made it onto buses.