First of all, relax! Your going to have a great time no matter what the crowd or weather. Universal Express is going to make your vacation! You'll be glad for staying onsite just for that!
The weather will be hot, but not unbearable if you plan your day accordingly. Same thing with rain. plan for it in the late afternoon and your will be fine. Go early in the morning. It is cooler. The general public does not have access to Express until 10 am, so its wide open on the lines. I did see more characters in the mornings, but their are usually lines. If you see a character out, ask his follower how much longer he/she will be out so you don't wait only to be disappionted. My DD has out grown the characters so I didn't pay alot of attention. We did Islands in the morning due to less heat and less chance for rain. Then it was either to the pool or over to Universal for the A/C and the inside rides. You will "feel" the crowds when you are walking in the later part of the afternon, but still little waiting with express.
Water rides will get you soaked, so bring a change of clothes if you plan on going to the A/C rides. You will feel a chill. What did I forget to tell you?