1. Yes, the standing area is all just one area. I'm not sure but I believe that 02 customers may get some early entrance rights... this certainly happened at Brixton last time... so if you want to get on the rail you may have problems... not really sure on the details though I'm afraid. Maybe check with the venue?
2. Not really sure about Gatwick, but generally I have found the 02 arena easy to get to. It's only a short tube ride from Waterloo station, and the station there (North Greenwich) is about two minutes walk from the arena, although do allow for a good 10 minutes to get from the main entrance to the actual entrance to the venue (you have to go past a load of bars and restaurants first).
3. I've actually found the 02 Arena quite good as a music venue. If you're standing at the front, or even a little way back, I'd say it sounds very good - not unlike a smaller venue. It is a modern building - a million miles from somewhere like Wembley arena, which is just a large concrete shed. I've had good views and sound from seating positions too, although fortunately never too far from the stage.
Personally I'd rather see Morrissey play somewhere smaller, but if you are going to do a proper arena then you could do worse than the 02. I found the Alexandra Palace a bit too echoey (?), and that was no where near as large. Anyway... bring on November!