Are wineries in Tuscany generally open on Sundays?
Can't actually respond to your Sunday question, but unlike Napa/Sonoma/Oregon/Washington, etc. they don't appear to be as much drop in. We've had appointments or went with tours for all of our winery visits in Italy.
I would check with the specific wineries you wish to visit.
In general, no. And many are appointment only even on work days. This article by my friend Michael Horne of Dall'Uva helped me plan a day in Chianti.