Europe is increasingly aware about access but ancient roads and streets and stairs make complete accommodation difficult.
Venice being a series of islands makes it particularly difficult despite being completely flat.
Here are some resources about access Venice:
https://www.veneziaunica.it/en/content/accessible-venice
https://blog.wheeltheworld.com/wheelchair-accessible-venice-italy/
https://www.introducingvenice.com/traveling-with-a-disability
https://www.sagetraveling.com/Venice-Disabled-Access
And other locations you mentioned:
https://www.sagetraveling.com/florence-accessible-travel
My dad was in a wheelchair for years so I have some frame of reference but we never braved Europe so I am extrapolating some here. Early on he used the wheelchair to avoid easy fatigue so he was able to stand and walk up short stairs or through an inaccessible doorway which opens up a lot of places that might otherwise have been off limits. If she can stand and walk to cross any bridges you might run into Venice should be pretty accessible but as you can see from the maps much of the city is accessible anyway.
It sounds like you considered instead getting wheelchairs locally and saving yourself the transportation issues of trying to take one. Check the forum since this has been discussed and I believe there was a reference to Florence providing free wheelchairs for all visitors to check out and return.
Regionale trains are often older and have stairs up into the trains but I would be surprised if Tenitalia didn't offer a list of stations and trains to help sort out the possibilities. The newer and more modernized fast trains are more accessible. On my last trip I did see several people making use of the Sale Blu service offered by the train system for assistance to the platform and onto the trains.
https://www.trenitalia.com/en/purchase/info_contacts/disabled_passengers.html
https://www.fsitaliane.it/content/fsitaliane/en/sustainability/people/social-commitment/accessibility.html
https://www.italiarail.com/station-services/are-there-services-travelers-disabilities
I have spent a fair bit of time in Verona and the core is flat and largely pedestrian with few cars during the day. That being said there are always the issue of cobblestones and ancient steps up into various places. But if your lodgings are accesible I think Verona is probably a good option. Verona is a very chic, arty city with a very cafe culture feel with tons of restaurants with outside dining where re-arranging a table to accommodate guests would not be a big deal at all.
I will also say that everywhere I have been in Italy there is great respect for people with limited disability. Even just having a cane people will voluntarily vacate the "accessible" seats on buses and line dividers that make your walk longer are often removed and lines are avoided without asking. Even on the tourist packed vaporetto to Murano and Burano an instagram couple made a beeline for the last two open seats together - because they were reserved for elderly and mobility impaired - and they were quickly shamed out of those seats and had to stand for the rest of the trip.
Hope that helps, have a great trip,
=Tod