According to Google Maps, it about 1½-2 hours from Rovereto (between Trento and Verona) to Malcesine by buses via Riva del Garda (at the north end of the lake).
I think the quickest way (7½ hrs) to get to Rovereto is to take a regional train back to Munich and one of the Eurocity trains (EC) to Rovereto. Those ECs run every two hours during the day and go to Innsbruck and over Brenner Pass to northern Italy.
With advance reservations (up to 91 days) the Bahn will sell you a Savings Fare ticket from Füssen to Verona for as low as 59€/P. These tickets are train specific (for the EC) and non-refundable. A fully flexible ticket for Füssen to Verona is 101,60€/P.
A slightly longer, but more scenic trip, would be to go from Füssen by bus to Reutte, then by trains to Innsbruck (change in Germisch-Partenkirchen) and pick up the EC there. That route goes via Mittenwald and Seefeld, Austria). However, because of multiple legs in Austria, the Bahn will not offer the Savings Fare nor sell tickets online.