The ITA flight from FCO to VCE is an award ticket, so if the Delta flight is delayed, I'll cancel the ITA tickets (booked through Delta) and just take the train. There are no return tickets, we are taking the train the rest of our vacation.
I started off thinking we'd do the train, but by the time we transfer to Termini, take the 4+ hour train to VCE ... taking the flight gets us to Venice three or four hours earlier, and for only 10k Skymiles each. We can spend the savings on a water taxi.
Appreciate the food for thought - it's easy to overlook details but unless something crazy happens with the flight across the pond, I am optimistic this plan will work!