I'm not sure what benefit having forced air would give. The hard part is keeping the airflow to a minimum. You want just enough air to keep the fire smoldering slowly.
A second cooking grate is always a good idea.
If you make a lid, a single exhaust is fine. My UDS uses 2" pipe, which is a little bigger than 2" ID. If anything, that's too big. It works best if I block it off a little. Make sure you can close your exhaust so you can save charcoal for the next cook. If you close everything, it should go out fairly quickly.
I've never tried a diffuser.
Your fire basket sounds about right.
I built one UDS with a BGE door. It burns evenly, but seems to leak air a bit, as it's not as good for low and slow. It's not on my UDS, so I haven't had to fool with it.
I'm very intelligent, but I mask it by doing stupid stuff.