one day Venice from 9am to 6 pm. Pls advice what & how sightseeing can be done

Instead of Mestre, why don't you use Santa Lucia station that is in Venice proper. Buy a 24 hour vaporetto pass and you can "see" a lot. You will of course want to go to Piazza San Marco, take some kind of skip-the-line guided tour. Otherwise with a vaporetto pass, ride around and hop on and off as you like, making sure to get back to the station in time fro the train to Florence.

I don't see any trains from Mestre to Florence at 7:00 pm, They are at 6:37 pm and 7:37 pm, and both of these originate at Venice Santa Lucia station.

If you arrive and depart from Mestre, you will have to get to Venice proper by train anyway.

Best bet for an overview might be a guided tour. Walks of Italy is a great company and has some nice offerings in Venice. https://www.walksofitaly.com/venice-tours

Otherwise, look at Rick Steves’ Venice guidebook for a couple of self-guided walks. He has some nice ones.

If you already have your tickets to Mestre, don't panic. Trains run every 15 minutes into Venezia S.L. It's a 10 minute ride.

Pick up a vaporetto pass that San recommended, (individual tickets are 7,50€) then take a ride down the Grand Canal.

For a good free tour, we have used them, check this group. If you use them, just remember to tip the guide.
Their site is a wealth of useful info.

http://www.venicefreetours.com/la-bussola-association/

You might consider doing both tours, which would give you a great overview of this beautiful city.

For skip the line tickets into St Marks, use this. The church is free, slight charge to go up to the museum but these eliminate a 45m
to-hour wait in line.

https://www.venetoinside.com/attraction-tickets-in-veneto/tickets/skip-the-line-saint-marks-basilica/

1. GRAND CANAL. Take a vaporetto (water bus) from the Santa Lucia Train Station to San Marco along the Grand Canal.
2. PIAZZA SAN MARCO: see the beautiful piazza, visit the Basilica, then go up the Campanile (bell tower, there is a lift) for a great view.
3. PALAZZO DEI DOGI (Palazzo Ducale) located next to the Piazza San Marco
4. Walk from Piazza San Marco to the RIALTO bridge. See bridge.
5. Walk back from Rialto Bridge to the BASILICA DEI FRARI.
6. See SCUOLA GRANDE DI SAN ROCCO museum, behind the FRARI basilica. Walk or take vaporetto back to station

I am not sure you have time for anupything else. It’s already a full day. I left out the Accademia Gallery (paintings) and the Peggy Guggenheim Collection museum, which are near each other. If you like museums and have time do those too, you could do those after no. 3 above.
Then you would do no. 6 then no. 5 then no 4 then go to station.