1) Wettins could do better if either Augustus II is more patient and less ambitious and avoids GNW, but easies options for them is just survive on the throne until Napoleon. Could happen if:
-Augustus III dies during 7 years war, coalition, like it was planned, support next Wettin during royal election.
-Frederick Christian, son of Augustus III, dies as child ITTL, so Francis Xavier (1730-1806) is elected and rules untill Napoleonic invasion. Say his reign looks exactly like reign of his father-he rarely even touches Polish soils, no drastic reforms, no 3 May Constitution. Reforms starts later, allowed by Austrians and Russians, who wants PLC to fight against Napoleon. Nappy invades PLC and beats it anyway, but finally is defeated like IOTL. Congress of Vienna restores pre-revolutionary order, including Wettin ruled PLC.
1) Reforms of Stanisław August were not too late, they were too fast and made in worst possible moment, without serious consultations with Russians. All PLC needed was to survive untill Napoleon, even in "vegetative" state.
3) Among other things:
-avoid Zamoyski's career to get too far, especially his disastrous idea of giving office of hetman (highest military commanders of PLC) for life should be avoided
-alternatively make Zamoyski devout supporter of Vasas and kind of Polish Richelieu, who really cares about his country, not only about himself.
-John Casimir have different wife and surviving son (and brain), so no civil war over Vivente Rege election of French duke and continuing dynastic rule in PLC.
-Ducal Prussian line of Hohenzollerns survives, Brandenburg and Ducal Prussia have separate rulers.