I’m not far from Corpoló today.
Take the freeway to Venice. Return the car at PIAZZALE ROMA.
Take the A14 freeway at the Rimini Nord entrance to direction BOLOGNA
then near Bologna you will switch to the A13 toward PADOVA and from there you will merge into the A57 to VENEZIA MESTRE. then, after exiting the freeway (toll freeway), you will go through the parkway across the Liberty Bridge toward Venice. Follow directions to Piazzale Roma.
Directions below.
https://maps.app.goo.gl/y7pFRHCqzrMbXYHn6?g_st=ic
Do not return at Bologna. It’s confusing to get to the station.
I also see no reason to return at Mestre and take the train. That is on the exact same way to Piazzale Roma. If you return to Piazzale Roma you go straight for another few km. For Mestre you would need to turn let on the overpass over the railway then backtrack to the station then find the rental office (if there is one for your rental company). Why complicate your life? The rental returns at Venice are at the Autorimessa Comunale (City Parking structure) and from there you can walk to the Piazzale Roma vaporetto.
PS: Watch your speed on the Liberty Bridge parkway between Mestre and Venice. Lots of speed cameras at work.