Turin to Lake Maggiore

Where on the Lake? Arona has a train station and you can get there from Torino with a change in Novara. I think Stresa and Baveno are served by Trenitalia, too.

And to get from Stresa or Baveno to Venice, you take a train to Milano Centrale and change there for Venice. Best to book that one ahead, for better prices (no need for Turin to Stresa, which I assume is on regional trains that cannot be booked anyway).

As stated, we need to know where on Lake Maggiore.

If like most RS travelers you are going to Stresa, I've done the trip from Turin to Stresa. I wanted it the easiest as possible. So, I took the train from Turin to Milano Centrale, and then switched to a train to Stresa.

Both Italo and Trenitalia operate between Turin and Milan. In Italian, the two cities are Torino and Milano, respectively. You will need that to get the schedule.

Between Turin and Milan, you have a choice of regional express and faster trains.

it is worth checking which Torino station your train will start in, and which station in Milano you will change trains.

Be careful that what you pull up doesn't make you take the metro or bus to change stations, and that your originating station is Torino is the one you expect.

We are thinking Stresa. I wonder if it's better to rent a car and drive? We'll be coming fromFlorence so we could even get a car there and drive to Turin. Is a day trip from Turin to Mont Blanc (Relais Mont Blanc/La Salle) a good idea?

A car in Turin is a liability, and the drive from Florence takes much longer than the train.
As for a day trip to Mont Blanc: it is a bit far and the views aren't as good on the Italian side, in my opinion. There are many interesting day and half-day trips from Turin that I would prefer, including Superga and Venaria Reale.

so we could even get a car there and drive to Turin.

You need to spend a little time on viamichelin.com and trenitalia.it/en to compare driving to train times. Without mentioning the ZTLs, the cost of parking, the motorway tolls, the Apennines and the uselessness of having a car in cities like Florence, Turin and Milan.
In Turin there is a Parking underground the ZTL, but you'd end up walking/busing everywhere while paying for the rental and the parking.

It's not eco-friendly, but a car may be useful on the Lake Maggiore, to visit Orta, and to drive from Stresa to Venice.

You could get the car at Turin's main station and drive straight to Stresa.

Or take a 2 hours bus from Turin to MXP airport, get the car there and drive to Stresa.

Last, Hertz has an office in Rho, a small town near Milan with a big exhibition centre. You could take a direct train from Turin to Rho and drive to Stresa from there.