As you know from your own involvement in this forum, most members are only interested in solving their own particular problem without a thought to giving anything in return, so it's always refreshing to see somebody return and offer their experiences.
Regarding the hit-and-miss musty damp air, have you checked that the condensate outlet from the heat exchanger is open? It is supposed to allow the condensed water to drain out, but can become blocked with dirt, leaves, mould etc. leading to damp air air or even water in the cabin. I have no idea where it is on a A6 or what it looks like, but there will be one. The outlet on the Ford I had many years ago was a steel pipe with a split rubber bulb on the end. The idea was for the split to open under the weight of water, and then close of its own accord, though I daresay many different designs will exist.