We just got back from DL..we were there Apr. 29th-May 3rd, and it really was a great time to go!
Thursday DL closed at 9, but Fri/sat/Sun DL was open til 12am. Fireworks happened Fri/Sat/Sun as well (not Thursday). I believe DCA was closing at 9 on Fri/Sat/Sun.
As for crowds...Thursday and Fri (until mid-late afternoon) were more than reasonable crowds. We pretty much walked on everything, or had a 10 min. or less wait. The weekend was a bit more crowded, but we still didn't wait long for much. 20 mins maybe?
with regards to heat, it was high 60s-mid 70s pretty much our whole trip. I was in jeans and a short sleeve shirt and not hot..DH wore shorts the whole trip and was quite happy. Sunglasses and/or hat are imperative for us though

I forgot my sunglasses, so we just had to buy a DL hat to make up for it...darn
We chose this time for the food and wine festival, and we loved it! The festival was SO fun, and kept us busy most of the day on Sat. and the morning on Sunday (Sweet Sunday= absolutely worth the price!). It's made us more excited to someday plan a trip to Epcot's F&W festival!