The seatbelt "beep" only runs for about 10-15 seconds before it shuts off - regardless of whether you've engaged the seatbelt. I agree that this beep is a royal PITA and I too would disengage it if I could easily engage it later.
The reverse "beep" will always come on when you're reversing. You will thank yourself that it does this because reverse and 1st Gear are VERY close and can be mistakenly chosen. In a hurry I've done it and caught myself thanks to the beep. It's been on the Celica GTS since it's induction in 2000 so I don't expect it to be removed (they share the same tranny).
Besides, it's a conversational piece...just scream out "WIDE LOAD" and watch your friends laugh and people look around.