As I said the Prussian army was a beast and had one of the finest commanders in history, comparing an army's quality against that (especially when commanded by a bumbling idiot like Charles of Lorraine) is hardly a fair comparison.
That's fine but the Russian army of the 7YW was badly organized, had rather mediocre commanders and generally ill-educated officers. Still, it won all battles except for Zorndorf (which was an extremely hard fought battle without a decisive Prussian victory even by Schliffen's estimate).
Austrian troops being decisive in winning at Kunersdorf
Because they did not participate in action until the Prussians had been defeated. Attack of Laudon's cavalry happened after the last Prussian attempt, a massive cavalry attack uphill against powerful artillery and stable infantry, failed with the big Prussian losses.
As for your argument on the Miracle of Brandenburg, I disagree with you on that,
Your agreement or disagreement does not matter. After Kunersdorf the Russian-Austrian relations kept soured preventing any serious joined action capable to destroy Fritz. Buturlin, appointed by Elizabeth after Saltykov got ill, simply refused to move anywhere but even prior to this Saltykov was vocally unhappy about von Daun.
Last but not least, you use example of Austrian defeats to say their army and leadership was bad (and I agree that the leadership was far from perfect) but both von Daun and Laudon were somewhat competent commanders, von Daun more so than you give him credit for.
Von Daun was very competent but he lacked an aggressiveness and, in this strategy he was supported by Maria Theresa who ordered to make a medal honoring him with a text "keep winning by procrastinating". Quite obviously the Austrian goal of reconquering Silesia hardly could be achieved by procrastination.
OTOH, Laudon (also very competent and talented general) was often blamed for an excessive aggressiveness which more than once got him into a trouble.
Additionally, you can't judge the Austrian army's performance without comparing it to the the French (supposedly the greatest land power in Europe) and Russian performances against the same opponents.
French army at the time of the 7YW was probably at its worst condition and its performance was pathetic. What can be said about an army in which an officer could just leave the field and go to Paris without a risk of being punished?
Russian performance is an interesting issue. As I said, the army organization was clumsy, the tactics was obsolete, its ability to maneuver was not very good, its officers corps was generally ill-educated and they did not even have competent top leadership (Apraxin was mostly a political animal, Fermor did not have previous experience of the independent command, Saltykov - the same, and the only good thing the contemporaries said about Buturlin was that he was handsome). The army had advanced field howitzers, "unicorns" (not to be confused with less effective "secret guns" of the same name), and that's probably was the only advanced thing. However, they managed to occupy Eastern Prussia and Pomerania, occupy Berlin (together with the Austrians), win the field battles against Prussian generals and in the direct confrontations with Fritz not to lose decisively in one battle and to win decisively in another.
Perhaps the best explanation "why" was given by a Frenchman who served as a volunteer in the Russian army during the 2 Ottoman war of Catherine II. He was comparing the Russian and Austrian armies trying to explain why a much smaller Ottoman force defeated the Austrians while with the reversed numeric odds the Russian troops would always beat the Ottomans. In his opinion the main thing was a "perception". The Austrians had better educated officers and better drilled troops but they did not have a "perception of winning" and treated the Ottomans as an equal enemy. As a result, when facing them, the Austrians would get engaged in a complicated maneuvering and lose. OTOH, the ill-educated Russians "knew" that under no circumstances they would be defeated by the Ottomans so at the sight of an enemy they'd just arrange their troops into the square formations and attack. With the adjustments to the tactical issues the principle probably applies to the 7YW as well. OTOH, when acting under command of the energetic general like Suvorov, Austrians were winning battles both during the Ottoman War (and the same Prince Coburg was defeated when acting alone) and in Italy so it is not unreasonable to assume that the issue was not as much quality of the troops but rather attitudes of their commanders. I'd risk to add: and the tendency to over-complicate the war (look at the general plan of campaign of 1805 and Weyrother's plan for Austerlitz).
Of course, the known pro-Prussian sympathies of the "Young Court" and deteriorating health of Empress Elizabeth had been an additional strategic factor. Also one needs to keep in mind that for Russians (unlike Austrians) this was a completely cabinet war.