Mehmet's Italy campaign succeeds, Ottomans go onto to vassalize Grenada, it might have a chance of survival
Okay, let's recap. The Granada War ran from 1482 to 1492. IIRC, Granada lost access to the Mediterranean around 1487. What are the Ottomans up to?
Mehmet invades Italy in 1480. You propose he succeeds. How long does it take him to actually conquer it? How long does it take him to pacify it? Suppressing rebellions and repelling crusaders will occupy a vast amount of Ottoman resources.
When Mehmet dies, Ottoman success in Italy is likely to lead to Cem receiving significant Western support. The civil war will be at least a distraction for the Ottomans.
Following OTL, Matthias Corvinus and Stephen Bathory in Hungary and Stephen III in Moldavia will be fighting the Ottomans in the Balkans. OTL, Matthias was able to force the Ottomans out of northern Serbia. ITTL, with the Ottoman forces being tied up in Italy, he will be in a strong position to put pressure on the Ottomans. Factor in the possibility that that Ottoman successes in Italy might motivate Poland to fight with (or at least 'protect') Moldavia rather than invading it, and the Ottomans start to get strained.
Now, while all this is going on, you propose that Bayezid is also able to send a force across the Med that can fight off Castile and Aragon? And in their own back yard?
For the Ottomans' sake, I hope the Mamluks are too occupied with communial singing to think about retaking any Ottoman conquests. Maybe the melodic sounds will keep the Qizilbash sufficiently distracted that they won't be able to find their red hats and rise up in rebellion.
All in all, I think your proposal is a tad ambitious.