1. Yes, in transparent mode it does.
2. Transparent proxy gives you power to do filtering, caching without even client knowing about proxy.
3. Yes, there is no order needed, you can install them at any order, all you need is that firewall should allow proxy.
4. I don't really understood that but if you are acing http_accel mode or something then its just to tell that can be used in transparent mode also. But many time people refer caching mode as accelerator mode.