This is a part of the world I know pretty well, and I'd consider a slightly different itinerary. And, as already pointed out, you need to add some time to this trip. I say this because you're looking at losing the equivalent of two days (minimum) traveling between your three primary destinations.
My experience is such that you will need full days for any tours of the Normandy sites. And two days is a minimum of time. This will probably need to be extended depending on when you plan to visit, especially if you aim at the week around the D-Day anniversary, due to the crowds.
Second, there is very little WWII "scenery" within Metz or Saarbrucken. Most of the battles for these cities took place outside the perimeter and, while there are places where monuments and such are placed, in many cases the cities have grown to cover the historic battlefields. For instance, the most prominent marker inside the Metz city limits is the one just outside the cathedral St Etienne thats shows the date the city was surrendered. If you can find an old copy of the British publication "After the Battle" for these cities they are a great reference, but even then the data and pictures they have are now 35-40 years old and can be difficult to use.
One thing you will probably note, especially away from Normandy, is that WWII is much less visible than either WWI or the 1870-1871 wars. That's because there was more direct French involvement in both of those conflicts in and around the area stretching from Paris to Saarbrucken.
I'd seriously consider a linear itinerary, either starting or ending via Frankfurt, with the other terminus at Paris. I say this because it would eliminate backtracking and the consequent loss of time to travel; it being faster to get from Frankfurt to Saarbrucken than from there to Paris. There's no need to fly in and out of the same airport.
The advantage of flying into Frankfurt becomes even more viable when you consider this allows you to eliminate days in Paris, as you can easily get from Frankfurt to Saarbrucken the day you arrive if you want to add another 2.5 hours travel. And Saarbrucken, or Frankfurt, will be considerably less expensive than Paris to overnight in. If any of your crew have to spend time in Paris it would be best to do so before, or after, the main objectives are completed.
Getting back to the WWII sites around Saarbrucken and Metz, there are places still visible, but you will require a car to find them. The Westwall (Siegfried Line) fortifications, as well as the French Maginot line ones are not easily reached via public transport, and in many cases will require some trekking. You can spend days looking for these; I have.
There's a bit more I could add, but this should give you some idea as to options. By all means, I highly recommend visiting Metz and Saarbrucken, but be aware that both of these cities offer much more history than that of WWII. Metz is one of my favorite places to visit, and between there and Saarbrucken, just outside St Avold, is the Lorraine American Cemetery and Memorial, where some of the men your grandfather knew ended up. https://www.abmc.gov/Lorraine