What PODs would make for the most successful Crusade of Varna?
November 1443: Skanderbeg defects directly into the Crusader army, recruited by Hunyadi as a commander.
November-December 1443: Murad II agrees with Kasim Pasha and Turahan Bey's harebrained plan to burn down Sofia and retreat to the mountains, but accidentally get trapped in the city. The three main commanders of the Ottomans in Europe die. The Ottoman Empire falls into a succession crisis, as Mehmed II is only eleven and Murad II is unable to return to the throne to save the empire.
December 1443: The Mamluks decide to assist the Karamanids in their assault on the Ottomans. The attack is strong enough to ignore the Ottomans' accusation of treason/apostasy in 1444 that stopped the Karamanid invasion, and also wears down the Ottoman army before the main showdown in Rumelia.
early 1444: Venice and Genoa decide to aid the Crusaders and commit to the war fully. They continue to blockade the Dardanelles instead of ferrying the Turks across, so the Crusaders may actually take Edirne before the Ottomans cross.
9 November 1444: Cesarini's idea to set up a Hussite-style wagon-fort is accepted, and Hunyadi goes on the defensive at Varna until naval reinforcements arrive.
10 November 1444: Wladyslaw III is able to kill Murad II in his initial charge. The demoralizing effect causes part of the Ottoman army to rout, leading to casualties on the Ottoman side as they are run down, causing more to rout. Hunyadi wins the battle and with Venetian and Genoese support takes Edirne (though would have to fortify against an Ottoman counterattack).