September is my favorite time to go! We have been going every year during Labor Day week. Starting on Labor Day, everyone goes home and the parks empty out. The longest line I can remember seeing is 45 mintues for Toy Story Mania. The old-school fastpasses for that ride would run out in the early afternoon. I don't know what it will be like with FP+ now. We are so spoiled by Sept that we don't like to wait more than 20 minutes for a ride. The weather is good. It will be hot and it will rain, but it won't be like July.
There are only a couple of things to watch out for. We have always found that the late EMHs in the MK get crazy crowded. I don't know where all of the people come from, because they weren't there in the day. Also, Night of Joy is held in the MK the Friday and Saturday after Labor Day. If you go with the dates you listed, you will miss it. If you go any earlier, just know to avoid the MK on those days. The park fills up quickly with the NoJ participants and closes early for the concerts.
As for the free dining, it has always been offered in Sept, but it depends on where you are staying. There are some rooms that do not come with free dining. We were told this is because those resorts are popular enough that they do not need the incentive. As long as you are flexible about where you want to stay, you can get the free dining.
