This has been some great feedback! Especially since there has been nearly 2 months worth of back and forth conversation/planning within my family that I am not able to post here.
UPDATE: While my father would like to see his ancestral home, he knows the difficulty of the location (Stroit is the town), he says his biggest "musts" for Germany are the typical Bavarian experiences: He wants the mountains and Oompah music/beer halls. The only big historical site he wants to see on the entire trip is the beaches of Normandy. With that in mind, here's what I think we'll do now: cut Berlin and cut Hanover and give a day back to the other two countries. Thus, our itinerary would like this:
UK: 6 Days (3 in York, 3 in London with one London day being a day trip to Oxford)
France: 6 Days (2 in Bayeux, one overnight in Mont St. Michel, 3 nights in Amboise. This is flexible as we may add another day to Bayeux or add an overnight in Paris)
Germany: 6 days (4 in Fussen as a base, or possibly Oberammergau thanks to the tip, 2 days in Munich, fly out of Munich back to the states).
In the UK, we'll just do trains and taxis. We'll take a Eurostar from London to Paris, then a train from Paris to Bayeux for the travel day. Bayeux, MSM, and Amboise will be via car rental. We'll drop off the car in Amboise, train to Paris, flight to Munich, then train to whichever area we decide on. After researching times and distances, I think this would work. It is a mix of fast paced, but also slower moments.
Oh, and to answer the other questions in terms of sights: Both parents are excited about the sites of York and London, particularly the museums. They are both lukewarm on France, but the beaches of Normandy are a big must for them. Otherwise, they just want to drive by the Eiffel tower while we are in Paris. The Loire Valley is a bigger deal for my wife and I. And for Germany, like I said above, the stereotypical Bavarian experience.