If you have checked luggage, you would basically need to be able to enter Germany if you have the separate ticket. How that works for your passport now, and how it will work a bit down the line is tricky to say. If everything returns to a normal, both Greece and Germany are Schengen countries and there would be no issues, but right now there would be border checks between Schengen countries and eligibility for Greece does not necessarily mean eligibility for Germany.

You should probably try to get a Lufthansa flight from Athens to Munich to make it all more easy.

By the rules, ANA does not allow for routing changes on awards.Though being an involuntary route change, I am not quite sure how they would handle it. My suggestion, find a specific star alliance flight from the same general area that would fit with your flight from Munich, and ask specifically if you can have that added indtead of the cancelled one.