Rental car question in Venice Mestre train station

Its impossible to say what the time at the car rental desk will be. There is another direct train at 10:38. You could also consider taking the 9:20 Italo.

Check with the office if you can drop off the keys before 8. I would want to be on the platform no later than 8:30, and you need about 10 min to gather your stuff and walk to the station and get on the platform (which will certainly entail walking through the underpass to reach track no. 2 or higher since trains in Italy travel of the left track and Track no. 1 will be for trains going to the opposite direction you are going).

Where will you stay the night before?
Could you drop car the night before?

Thanks for all the good (and quick!) responses! I am staying in Verona the night before and plan to drive early to get to Mestre when they open at 8.

Consider returning car in Verona

If you are in Verona then return the car in Verona. Why back track to Venice to go down to Naples? It makes no sense. If you don’t want to return in Verona then it would make more sense to return the car in Bologna than Mestre. But in your shoes I would definitely return the car in Verona as soon as I get there (no need for a car in Verona, the historical city center is pedestrianized and forbidden to cars except residents with permit), then take the train from there the next day. Most (all) fast trains may require a change (in Bologna, generally) but it would still be a better option than returning the car in Mestre.

Thanks to all for your great replies. Super helpful! For a variety of reasons, we don't want to drop the car in Verona. However, Roberto, your suggestion to drop it in Bologna is excellent. Didn't occur to me. I think that is our plan now. Many thanks!