Since the car has been offroad for some time, it might be worth checking out the following thread on the drains:

https://forum.a8parts.co.uk/showthread.php?t=16008

As for the dampness at the rear of the passenger footwell, it might be possible that the water is getting in from the O2 sensor grommet, which is located in that area, especially as Botang's hole and A/C overflow blockage would soak the front of the passenger footwell.

My previous A8 used to get water coming through the rear passenger air vents, behind the central armrest, I believe caused by blocked drains somewhere, but never did resolve as the gearbox failed shortly after this issue and it was scrapped anyway

I believe water coming through the rear vents is caused by blocked drains from the HVAC system. It's the orange drain pipes either side of the transmission tunnel in the thread linked to above.