Just finished reading the TL. Intriguing premise and scenario.
But shouldn't the Christians be doing much better in the Balkans by this point? IOTL the Austrians pushed much deeper into Ottoman territory than ITTL. With a coalition this big and no worries of French intervention re-conquering Constantinople should be well in the cards before invading the Holy Land.
Looking at a map, it is a reasonable question to ask why the Coalition would be making less progress against war targets within 1000 km of the nearest member's capitol, compared to war targets in the eastern Med that are over 3000 km away. I think there would be two ways to explain this.
First, logistics. In that era, like most others, it was cheaper to large objects such as barrels 1000 km by sea than 100 km by land. Armies at that time, assuming they were not moving through territories like a horde of locusts, despoiling the land for supplies, tended to move only at the pace of their supply train. Its would be easier and cheaper to campaign in Egypt/the Levant than the the Straits, at least until such time as the Turks naval power was completely eliminated.
Second, politics. Any gains in Balkans, the Hapsburgs backyard, would been seen as pure wins for Austria, and less so for France, the Dutch Republic and others. Any economic gains to the parties bordering the Atlantic would be limited. The Levant, however, would be seen as more of a win for all parties. Being close to the sea, they could easily develop commercial and political links to more faraway European powers. There would also be a stronger romantic appeal to campaigning in the Holy Land for the non-Habsburg powers. Britain, France and to some extent even the Dutch had glorious historical narrative to draw upon in the region, especially for the
people actually making political decisions. The greatest British and French families included many crusaders.
Also eagerly awaiting an update!