Our plan for the day was very similar to what you are interested in. We had our ceremony at 4:00, at that time they did weddings on the even hours and rehersals on the odd at the WP. That may have changed, I'm really not sure. We had time to do our ceremony, pictures, dinner and about an hour of dancing before we would have left the Living Seas for the Illuminations party. There was a schedule change that ultimately added some extra time in our plan but the EM did something special for the extra couple hours we had. We opted at that point to end the evening so that it didn't run too late into the night, but we knew we had other things we wanted to do (we did Theme Park pictures, you have to get up really early for those!) so we wanted to wrap up before it was late.
You might want to call ASAP and set up your date, especially since you have an idea of where you want to have your event. They can check dates and availability and reserve these until your SM/EM is assigned. If you are thinking September of this year you will have plenty of time to get everything (officiant, flowers, food, music) arranged but may have a challenge getting every site you have your heart set on for the same date, they will work with you as much as they can. We ended up having our event on Friday because Saturday the UK was already booked for Illuminations.