Yes, you got it. The "victory" that never was is Vernon's assault on Cartagena de Indias in 1741. During the early stage of the battle, when the British victory seemed certain, several different commemorative medals were minted in London to celebrate the capture of Cartagena. Unfortunately for Vernon, later the "victory" turned out to be a big defeat. Over to you, pb. Regards.
Don Juan de Austria (John of Austria)? He was 24 years old at the time of Lepanto (though the year of his birth is not totally sure), but on the other hand he has been appointed as Captain General of the Sea when he was 21 (IIRC), and he was just half-brother of king Felipe II.