Trouble is, the 1866 war isn't an easy one to stalemate. The Prussians are on the wrong side of a mountain barrier, which they had to negotiate narrow passes to get through. So if defeated they are in a trap, rather like the French at Sedan. By the time of Koniggratz they also mostly hadn't eaten for a couple of days, and would have been retreating over land they had stripped on the way in.
Seems to me that if you don't get a complete victory for Prussia then you get a complete victory for Austria. Stalemate isn't really on offer.
On another thread, this was said with regard to the Seven Weeks War.
So, let us assume a crushing defeat for Prussia in Bohemia and the capture of lots of her troops. The Prussian victories against the smaller german states will not matter a bit, but will have still the effect of having crushed all ideas of a viable "Thrid Germany".
What will the Austrian demand be, what will they probably get ?
Not just territorial, but also political - will they even try to create a Greater Germany? I am not quite sure whqat the Austrian minister president Belcredi thought of the matter; in any case he opposed the 1867 Ausgleich and favored a federal approach based on the old principalities and counties, not an ethnic borders, a quintuple monarchy: German Austria, Bohemia-Moravia-Silesia, Galicia-Bukovina, Hungary, Croatia-Dalmatia(-Carniola?). In this TL (especially if some Magyars tried to rise up in support of Prussia and were defeated) he might be able to push this through. BTW, if even parts of Prussian Silesia are annexed, this B-M-S will probably have a slight German majority, too.
And what will happen in Prussia? Wilhelm I will abdicate in favour of Friedrich III, I guess, and Bismarck will be dismissed in disgrace, but beyond that? Of course that mainly depends on the losses they will suffer. Probably the loss of Upper Silesia to Austria is a given, but what more?
France will be bought off with Veneto (and parts of the Prussian Saar area - borders of 1814?), I guess.
Russia will initially dislike too much losses for Prussia. But if Friedrichs accession will be obvious before the treaty, they might even prefer to see *his* Prussia severely weakened.
What will the UK do, especially since Princess Victoria is now Queen of Prussia? Since Friedrichs liberal ideas are well known, I guess they will oppose a the complete dismemberment of Prussia as well.