Genoa and Venice guaranteeing Byzantium? Even if it would be accurate historically, It hurts the gameplay too much. It is something we don't need at all, especially for a nation like Byz which is designated to fall in the first years of the game. Maybe making some events like "Republic of Genoa/Venice sends help" or something like this would work.
P.S. Here is another 150 years off topic war if you are interested
https://en.wikipedia.org/wiki/Byzantine–Venetian_War_(1296–1302). What I was trying to say is that Venetian and Byzantines were never friends, that's why I highlighted "historical friends with Venice" in the quote.