Point to Point by Train

My friend and I have just decided to cancel our rental car for an upcoming trip to Italy. We’re now trying to decide how to get from Fussen, Germany to Venice in one day...we’re dropping a car off in downtown Garmisch. What’s the quickest way to Venice? Garmisch was the one most southern city that didn’t have a drop off fee.

For two weeks, we’ll travel by train...is it reasonable to buy our tickets on the day we’re leaving, say Venice to Florence or Florence to Siena? I’m really uncomfortable without a car, but I understood that parking is a nightmare in Italy.
When we’re leaving Rome, I’d like to pickup another rental car in order to drive around Tuscany, and continue to Cortina. The Dolomites are our last stop. Is there a rental agency that will rent in Rome and that doesn’t charge a drop-off fee for northern Italy? If we dropped the car off in Cortina, how could we get to Munich the same day? Can we drive west and find a city north of Bolzano to drop off the car and catch the train north to Munich? Our hotels have been set in stone and are non-refundable....2 nights in Cortina...then, originally we were going to drive through the Dolomites and be in Munich for our last 2 nights of our trip. Any advice please? Thank you!