What happens if the aircraft spends the first two hours of the flight going through turbulence and the seat belt signs don't go off? Surely the aircraft isn't still "taking off"?

I would think once the crew get out of their seats then the take-off rules no longer apply. You should certainly be able to recline your seat then, which you can't do during take-off. Turning it into a bed without removing your seat belt would be a bit tricky, but maybe just doable in Club although probably not recommended.