how best to optimize 2 day visit to Utah Beach and Mont St Michel

Stay in Bayeux both nights and day trip to Mont St-Michel. That is what we did and it worked well. We visited all the US beaches. We had another day and also visited the British and Canadian beaches. My father landed at Omaha Beach. We stayed at Hotel Lion d'Or in Bayeux, historic. All the famous generals, statesmen from WWII era stayed there. It is still owned by same family and they are helpful to people like us who have a family connection. We enjoyed looking at their scrapbooks from WWII.

Staying in Bayeux is a good recommendation. I would also strongly recommend that you NOT drive straight there from the airport. Jet lag and driving don't mix- you're in effect driving impaired. Take the train instead. Rent the car when you get to Bayeux, if you must. Personally, I think the small bus tours are of greater worth, given how informative they are. Unless of course you have a military history background. Day trip to MSM the next day and perhaps finish by checking out the Museum and the tapestry in Bayeux. When you check out of the hotel, ask them to hold your luggage until you're ready to head to Paris.

The drive back to Paris isn't that long. Rouen isn't that far off the A13, the most direct route. If either of you love gardens or Impressionism, Giverny (Monets garden) near Vernon, would be another good side trip. In either case, make sure your luggage (and anything else you won't be taking with you) is well out of sight when you park your car.

Your proposal for the first day sounds very difficult, perhaps superhuman, at least if you arrive on an overnight flight. You are unlikely to be finished with immigration, luggage retrieval, etc. before mid- to late-morning. Then there is the process of getting a rental car. Google Maps says the drive to Bayeux is at least three hours, and you don't know the countryside. It's getting to be late afternoon, and you are proposing to do a worthwhile tour of the Normandy beaches, or even just Utah, in the few hours of daylight you are left. Physically you might be able to do it, but I would be capable of only two things: a meal, and bed to catch up on sleep. My plan would be to take the train to Caen, settle in, and then a quick train commute the next morning to Bayeux for a guided tour that would do justice to the amount of history to be understood (and which your father was part of.) Or take the tour from the well-regarded Caen peace museum. Leave Mont St-Michel to another trip; there are other things to see in Caen and Bayeux.

Having taken a lot of overnight flights to Europe and seen how other people look upon arrival, I'm convinced that the vast majority of US travelers are not fit to drive 3 hours on arrival day--though certainly some of those folks think they can. Take the train to Caen or all the way to Bayeux where you can join a van-tour of the D-Day area the next morning.

I DO NOT recommend this, but Google found a one-day tour from Bayeux that hits both Mont-St-Michel (during the tourist-clogged daytime, of course) and some of the D-Day sites.

Especially with the family connection, I urge you to give the D-Day visit the (minimum) time it deserves, which is a full-day tour.

Be aware that many of the one-day or half-day tours do not include Utah Beach. If you already have some knowledge of the war, and have specific places to see, you can easily do it on your own. It was a very large battle, and a very large area. Even a well-run tour might not cover what you are most interested in. BTW, the museum at Utah Beach is, in my opinion, one of the best in the area. There is also a little café there by the beach. Or you could drive into Sainte-Mère-Église for lunch. The drive will take you over one of the four causeways that the troops landing on the beach took inland to meet up with the paratroopers. Ste.-Mère was the center of the airborne operations, and the town really celebrates the U.S. veterans.
As far as driving vs. train, many will caution against a drive after an overnight flight. I have both driven and taken the train many times. It depends on how you fly, and how you recover. Also, having another adult who agrees to share the load and keep each other awake, vs. just falling asleep on you might make a difference.

Stay at MSM on Sunday night. That is: slow down, spend your first 2 nights in Bayeux, get to MSM in Sunday afternoon, depart for Paris on Monday morning.

This itinerary lets you avoid leaving luggage in the car.

You can drive back to Paris, but at least investigate catching the TGV from Rennes, which will save you some time. Are there interesting things to see on the road back to Paris? Sure, but you don't have time.

As others have said, this plan is too ambitious. Skip Mont St-Michel and concentrate on Utah Beach. I would contact the museum in advance and let them know your father landed there. They may be able to give you a special tour or show you items in their archives that are not on public view. It is an excellent museum, so be prepared to spend half a day there and walking along the beach. As for driving from Orly to Normandy, it can be done, but it's better if you have driven in France before. If you do drive, go straight to Bayeux and relax for the rest of the day. The next day do Utah Beach (and Ste. Mere Eglise or the American cemetery at Omaha Beach if time). Return to Paris on Monday.