Frankfurt-Passau: Look up whether your airline is participating in the Rail&Fly program of the DB. So you would be able to hop on any long distance train any time on arrival in FRA.
Neuschwanstein / Dachau: Buy local tickets on the spot (Bayern Ticket for the Neuschwanstein trip, MVV ticket for Dachau).
München - Nürnberg: You can travel on a fast regional train every other hour, usable with the Bayern ticket (€31/2), the first one leaving at 9:01. Alternatively, pre-book a train specific Sparpreis ticket for the ICE (saves approx 40 min).
Note: there are three regional lines connection Munich and Nürnberg: a. Munich - Augsburg - Nürnberg, b. Munich - Ingolstadt - Treuchtlingen - Nürnberg, c. Munich - Regensburg Nürnberg. I'd avoid those, since trains can take three hours and more. Use the RE train running over the direct ICE line München - Ingolstadt - Nürnberg (bypassing Treuchtlingen), aka "München-Nürnberg Express".
Nürnberg - Frankfurt: pre-book a train specific Sparpreis ticket for the ICE. Do it right now since prices will go up.
What's your point of origin on 5/28?