Hi, just dropping by too. A few remarks.
Vincentello d'Istria was appointed Vice-Roy in 1416 by Alfonso V. In 1419 he already controlled the whole Island, except for Calvi and Bonifacio.
In 1420, the Aragonese took Calvi and besieged Bonifacio as has already been said but Alfonso had to leave in January 1421 for Naples where Queen Giovanna called him.
Calvi, the genoese capital before Bastia, immediately revolted against the small aragonese garrison and Vincentello was never able to take these two cities again. He had his own capital in Corte though.
It seems that he was not a very good ruler and in 1430 a first round of revolts lead mostly by genoese partisans almost succeded. A second round in 1434 forced him to leave the island to get some help, he went to Tuscany and when sailing for Aragon was captured at sea by the Genoans who had him tried and executed.
The leader of the revolters, Simone da Mare, then governed Corsica for the Genoans until 1436 when he was defeated by other revolters, mostly Vincentello's partisans. Genoa sent an expeditionary corps in the island who finally took control back in 1437, for a few years only.
I have some events with an independent Corsica, vassal of Aragon under Vincentello d'Istria in 1419. The island becomes vassal of Genoa under Simone da Mare in 1434 and is finally inherited in 1436.
Of course the 1434 event has an option to remain independent and another one to remain under Aragon. This is not so unhistorical as a powerful lord was in fact elected Count by a local assembly in 1436 while Vincentello's nephew was appointed Count by Alfonso V in 1435.
Then there are a lot of events for Genoa with revolts and revoltrisk until 1511.
Would that solution be acceptable?
P.S. I know nothing about the exchange with Visconti, but the agreement certainly did not hold because Alfonso V made another attempt against Bonifacio in 1424 and continued to support the feudal lords opposing Genoa until his death.