Never heard of a roof rail issue as cause of water ingress on A6 or even A4 Avant, but never say never.
If no panoramic, then that kind of rules the obvious culprit which I’ve dealt with before, however if the car has been subjected to heat, and I know there’s been some daft temperatures down south, then anything is possible.
The fix won’t be straight forward, but leaving it, then you risk mould, damage to electrics, airbags and interior trim, etc.
Wet seatbelts, is that at the mechanism lower down or higher up? The reason I’m asking is if there is wet staining higher up, you have no choice, you’ll need to drop the headlining.
Prior to doing any of this, rule out blocked plenum drains. They live behind the bulkhead behind the engine under the wiper mechanism. Also check the rear spare wheel area where the space saver and amp lives, including both rear arch compartments.
If it is leaking above, then you’d likely see condensation in the car. To drop headlining, you need to detach battery leads in boot, leave 30 mins. Most tools are anti-marr plastic trim tools, Torx screwdrivers, plus special tool for interior grab handles. You need to work clean. Light grey interior trim marks very easily with dirty hands, so clean tools, nitrile gloves, etc.
I can post more details on the headlining drop but confirm some details first. The trouble is it’s labour intensive and most garages aren’t set up for this type of work, it’s really more a body shop thing.