There is no Miracle of the House of Brandeburg. Elizabeth lives longer, Frederick either dies in battle or suicide. Russia and Austria complete conquest of Prussia.
1. Is it plausible Russia and Austria sign separate peace treaties with Great Britain?
Assuming they do, Sweden gets Stettin, Austria recovers Silesia, and Russia acquires East Prussia. Russia then trades East Prussia back to Poland for the Duchy of Courland (and parts of Belarus?).
France is alone and signs over all of New France or parts up to the Mississippi River.
2. Would Austria try to install the Duke of Parma in Austrian Netherlands and make it a French vassal in exchange for helping in the next Bavarian succession?
Without Prussia to keep Austria away. It seems it is inevitable that Austria gets Bavaria in this timeline.
Poland-Lithuania continues to be a Russian vassal state. No Prussian pressure to connect Prussia to Brandenburg forces Austria and Russia to join in a partition.
Now Augustus III dies and there is an election in Poland. Without Prussia, they supported Russian interests in the election to achieve alliance, there would be competition for the throne. Likely Austria would produce a candidate against Stanislaw Poniatowski. Russia would be outnumbered against Austra and likely France, Great Britain, or Ottomans(They wanted a weaker Russia).
3. Would it be plausible that the Commonwealth would survive to the Napoleonic Wars in this scenario?
4. In the event that they do survive in some state to Napoleon. Would they join the Allies against Napoleon? Would the revolutionary spirit find sympathy among the Szlata or middle classes?
Any other butterflies do you see from these situations?