Italo Train schedules/ticket availability for late April/May

They have never set a fixed date. It think it's no coincidence that they blocked all bookings the week before Easter, when "the season" begins.

I'd just wait some weeks and check again, otherwise you can always try to post the same question on their Facebook page.

Please go to loco2.com or trainline.com. I am not sure what the problem is with the official Italian sites, but I just tried a sample booking on both for May 8 (also your earlier dates) for Firenze-Venice, and it is working all the way through. Both are simpler sites to use, (I last used Trainline over the weekend for two trips late April), there are no fees, and the price is in USD and at the appropriate conversion at purchase. And on Trainline I was able to get a combination from Ravenna to Venice that did not show on the two Italian sites, a combination that did not just go back to Bologna and was at a better timing for us.

There is no problem on the official italian sites, if re-sellers like loco2 and trainline sell italo's tickets before italo... they are just selling tickets that do not actually exist. It's not rocket science, re-sellers do not decide the schedules.

When they sell tickets from 2 competing companies to take you from A to C via B... do they warn you than in case of delays on the first train, the competing railway company running the second one won't protect you?

I bet that looking for a train to "Venice" or "Cinque Terre" is easier when you are in your living room, unfortunately in Italy all signs and monitors will say Venezia S. Lucia, Venezia Mestre and the name of the 5 villages in the Cinque Terre. Not a re-sellers' problem, they are not supposed to help lost tourists wandering around train stations with a puzzled face.

Not all Trenitalia tickets are necessarily online yet for that period, either, though many are.