I don't think I understand how the Saxon Capitulation Event is triggered. So far I thought the Austrian player could prevent this event from firing by controlling one of the access roads to Pirna (Dresden, Radisberg or Dippoldiswalde). I also thought the latest this event could fire was late December 1756...
In my current game I managed to relieve Pirna, marching to Radisberg on the right bank of the Elbe with Browne, Lucchese and Kollowrath a(dn of course Hadik and Gaisruck). All of that to support Rutowski. I then marched part of the Austrian Army across the Elbe to Pirna. From there I advanced to Dippoldiswalde and only withdrew my rearguard from Radisberg and then Pirna once I was firmly in control of the pass back into Bohemia. By the first snow and blizzard may Army was withdrawn into the pass and was ejected from the mountains to the south. By late December I had withdrawn the entire force (Browne, Lucchese, Kollowrath and Rutowski) to the outskirts of Prag. I was still listed as in control of Dippoldiswalde though I no longer had a force there (I was relying on General Winter). So I was a bit shocked that with the early January 1757 turn the Saxons surrendered and I once again had a leaderless Saxon Army (in almost mint state) under my command (in the meantime the Prussians had relaid siege to Pirna in which I'd only left a skeleton garrison (garrison, supply and a single battery)...
Obviously I was only able to pull all the above off because the ai found it more attractive to try and attack Prag by slipping around my left, rather than assure the Saxon Army was destroyed or Browne be given a mauling. Of course they failed as Picolomini was able to defend that city (with the aid of the Saxon guard and Grenadiers which I'd evacuated by boat from Pirna)...
P.S.: Getting a bit frustrated by this event. I'd be happy if I could only save the Saxon Guard, Grenadiers and some cavalry with the leaders to command them. Ruttowski would be an added bonus, but not necessary. And the Saxon Line isn't that attractive to me. For now I'll just reload the previous turn and edit the save game to prevent this event from firing again...