Hi, dis4me. I'll try to answer as many of your questions as I can!
July 4th weekend will be crowded, but not unmanageable with Fastpass and breaks mid-day. I see your flight is booked for the last week in June. That would be a much better weekend to spend at DL if you could swing it.
It will be hot, but nothing like Orlando. It should be in the low 80s during the day, and in the 60s at night. The humidity is very low, so it won't feel sticky, and there shouldn't be any rain at all.
Plan on at least 3-4 full days in DL and DCA. If you're going to be in the area for a while, you should spread out the days you're in the parks so you don't get overwhelmed. The park hoppers are usually good for about two weeks after they're activited, so there will be plenty of time to see other sites in the LA area while touring DL. You might want to go to DL during the week if possible instead of the weekend to avoid excess crowds.
We usually spend about 2 nights in San Diego, but I'm sure you could easily spend a month there and never run out of things to do. Highlights include Seaworld, the Zoo, the Wild Animal Park (outside SD),
Legoland (in Carlsbad), Seaport Village, and Old Town. It's fun to stay in a hotel on the beach, and you'll want one that is pretty centrally located so you can be close to all the action.
Have fun!
