We are WDW vets, that have gone to DL at the begining of June the last 3 summers. We love DL! You will have a great time! Here are my opinions on your specific questions.
- How many days would we need to do everything in both DL and DCA?
Do you have small children with you? That can make a difference. DD has been at ages 4, 5, and 6. The first time we stayed 3 nights and got to the park at arrival day by about 2 p.m., and left the parks on departure day at about 1 p.m., so we had pretty much 3 full days. We did not feel like that was enough. The last 2 years we have increased it to 4 nights, with the same kind of park time on arrival and departure days, and it is better, but we still aren't every ready to leave Disney, and there is always more we wish we could do. We do like to spend the last half of the week visiting the beach and other Southern California desintations though. If we didn't have a smaller kid, I think we could move faster. As far as the major dining highlights, there is just no way to fit it all in. If you like character meals, I would highly recommend Minnie's breakfast. It puts the WDW character meals to shame. We have done it 3 times, and the lowest number of characters we have met during breakfast was 7, with the highest being 11. It is great random mix of characters too. We go around 10:00 a.m. so that we get some great park time in early, and then the restaurant is less crowded and the characters spend a ton of time with DD. I have fond memories of DD doing the macarena with Captain Hook!
- What about transportation from the airport - no Magical Express?
We fly into and out of John Wayne, as it is much more laid back and easier than LAX. It is also closer to DL, if I'm remembering right We do rent a car because we use it the second have of the week to travel to the San Diego area. It is kind of a waste the first 1/2 of the week, because we usually park it at DL when we arrive and don't see it again until we leave. I'm pretty sure there are services (other than cabs) that will take from from JWA to DL and back for a fee, but someone else can tell you the specifics of that.
- Is any one of the three hotels recommended over the other?
I would chose DLH or GC. They completely immerse you in Disney property. From DLH, you have a nice walk through Downtown Disney to get through the park, which we really enjoyed. GC, you actually have the entrance to CA right at your hotel, and you also walk through DTD to go to the DL gate if you want to enter that park and don't want to go through GC to hop to do it. Another point is that you definitely want hoppers at DL. It isn't like WDW. We have literally gotten from the back corner of DL to the back corner of CA in right about 10 minutes. The walk between the parks is across a plaza and is shorter than the walk from most WDW parks to the bus stops. Also the bag check is at a spot at the end of the plaza and is for both parks, so you don't have to mess with bag check lines getting from one park to another. We would go back and forth every day.
- Are some of the things from WDW (FastPass, PhotoPass, etc) the same at DL?
Yes. The photopass price is cheaper, but you also won't see as many photographers.
- What time of year is best to visit for a week? Should we avoid weekends?
That I don't know because we have been in the same time period the last 3 years. DH and I also were there once on a business trip for one day, when DD was 2 (Shhh don't tell her), and that was in July. We found it managable even in July. Some people will tell you to avoid weekends. The week we have been there, the annual passholders were blocked on the weekends, but not during the week, and our experience all three years has been that the weekends were the least crowded time, and it got pretty crowded on the weekdays because of the AP holders, and because the AP holders were going to get more and more blackout days as the summer went on. I don't know if that is typical for the rest of the year. If you are going at other times of the year, one thing to consider is unlike at WDW, sometimes the nighttime entertainment such as Fanstasmic and fireworks, only runs during (or around) the weekends and not in the middle of the week. Fantasmic is much better at DL, than at WDW. They have a terrific Peter Pan and Captain Hook section in their show.
Hope this helps! Hope you love DL as much as we do!