OK- Delta does not actually RENT any cars. They would book you with one of their partners I suppose.
I would forget that idea and for sure drop the train to Mestre- bus to airport to pick up car- that is way too convoluted!
Do your own booking with AutoEurope.com. They have a 35% sale going on now.
They are a US based consolidator and their customer service is stellar- that will be helpful in your situation.
Only book with one of the major agencies- Hertz, EuropCar, Sixt, Avis/Budget.
Book the ZERO DEDUCTIBLE insurance- then you don’t have to worry about anything, just "walk away".
A quick search shows me that the Hertz location in Venice at Piazzale Roma is open on Sunday til 12:30.
Go to AutoEurope.com
Search for Pick up Venice All Locations- same Drop off
Put your Pick Up and Drop Off time 11:30 AM.
All the various agencies will pop up. Scroll down to Hertz. Pick your type of car- we usually go with a Mid Size. Make sure you check Type of Transmission. Automatic will be more expensive than Manual.
You can look at all of that online and book online OR
Call them at their Toll Free number to book. You will have to pay upfront but this can be canceled or changed up to 24 hours prior ( check that).
They are very helpful and very knowledgeable.
You will take your fast train from Florence to Venezia Santa Lucia- arriving here by 10:00-10:30. Walk out of the station- (marvel at the Grand Canal in front of you ) and go to your right- Walk across the bridge to Piazzale Roma and find the Hertz agency.
Pick up your car and go. When you return - drive back to P Roma-there you are- already in Venice.
Be careful on the bridge driving in and out of Venice- speed cameras!
Ask for more specific help with how to get from station to P Roma. Hopefully you each just have ONE suitcase, carry on size to deal with.
If there is a train strike - you will know at least a day or 2 in advance- you would have to book a car from Florence- Pick up at a train station location (Borgno Ognisanti- this is where AutoEurope toll free customer service will come in handy), drop at Piazzale Roma.
If you are late and don’t get to your pick up by 12:30 closing time they will simply close and you don't get your car. There are no extra fines or fees for not picking up a reserved car.
You will be charged by whatever terms your contract states.