...but luckily I did not end up drinking to much water. Recently, I completed my own drill in open water. I can now certify myself, so to say. I did the training of the final round in 8 to 9m, 10°C water temperature: switching between full face mask and half mask and backup reg, and afterwards back again. Now I remember why I'm recently started diving with a full face mask.
Interestingly, two other divers with full face masks were also just about to dive when my buddy and I arrived at Lake Murner. They are both with the German Water Rescue Organization «Wasserwacht». To cite one of them: «no more diving without full face mask». That's a deal! I'm glad, I'm not alone diving a full face mask... The two from the Wasserwacht were diving an OTS Guardian mask and an Ocean Reef mask (without any integrated lamp).