pickup/return rental car on closed Sunday. May 24 (from Paris somewhere)/May 31(LeHavre?)

I see all rental car companies in France are closed on Sunday.

Simply not true. Airport rental offices remain open, many train station offices are open on Sunday as are many offices in principal cities. There are many in-Paris rental offices open on Sundays. Which ones specifically depend upon your chosen provider.

Why would you not drop off the car at CDG. Many companies such as Europcar or Sixt do not charge for a different, within France, drop off point. Hertz and Avis typically will charge to change the drop off point from that of pickup.

What time is your Sunday departure from CDG?

What Tocard said.

Beware that driving out of that station, and finding your way, is difficult.

If you are driving to Normandy or anywhere along A13, Versailles for example, all you need do is travel south from Montparnasse to the périphérique from where A13 begins. Driving anywhere, take a look at the route in advance and have a general idea of where you are going.

Admittedly, the shortest and simplest route from Montparnasse to the périphérique is not the most obvious, so I am not going to detail it here. However, you can always take blvd Montparnasse west, turning south (left) at rue de Sèveres and follow it to the périphérique. This is a very simple route but do not attempt to wing it with a GPS only departure. Study and know the route in advance.

There is absolutely no traffic Sunday mornings in Paris. This is the perfect first time to drive in the city.

Departure is 4:30 PM Sunday CDG. Great to know we can pick up and drop car off Sunday.
We will rethink our itinerary. Maybe skip night at MSM/ Stay in Bayeux 3 nights.
Then Honfleur and area. 2 nights. Etretat looks like a must.
Please excuse if redundant. TIA

Spend one might at MSM. There's no need to spend more than one night there (in fact, you are better off spending less than 24 hours there - arrive in the afternoon, spend the night, leave late morning....that way you miss the crush of humanity that sweeps over the place mid-day with all the day-trippers). So do spend one night, but definitely not two.

You can drop a car at Caen during closed hours but you then don't get signed off as having an undamaged, gassed up car and that is always risky. We did it once during their two hour lunch hour on a regular day and didn't have issues.

I would not drop MSM -- we once stayed there one night, two in Bayeux and then two in Honfleur with a day trip to Etretat and it was lovely. We did an Overlord tour of the American beaches on the full day in Honfleur. You could of course drive your own D Day route.

When I search for pick up/return in France, it's not unusual to see nothing available at noon. Even in big cities, typical hours are 7-11 and 1-6. If you search at noon, as an example, there are few options. Search at 10 am or 3 pm and you'll do better. Sundays do have more closures but train stations and airports will usually have open hours (while still taking a long lunch).

I picked up a car at the right bank train station in Rouen on a Friday. They had lunch from 11:30 to 1:30 but were open early morning and into early evening.