Second question first: I am not absolutely sure about an international Savings fare, but I am sure (it happened to me) that with a Dauer-Spezial fare (inside Germany), if your connecting train is late, they will put you on the next available train. But don't just get on the next train; go to the ticket counter and have them stamp the substitution. I can't imagine that it would be different for your case. Of course, if the delay happens in Italy, I am not so sure. Or if you miss your international train because the train into Milan is late, then...?
As for any regional trains to get you to your final destination (i.e., Mainz{?} to Bacharach), there is a clause (Vor- und Nach-Lauf) in the "Conditions of Carriage" that allow you to use any regional trains, until midnight, that you might need to get to your final destination.