We are planning to visit the Mosel/Rhine areas in Germany, then the Normandy area in France. We will have rental cars in both countries. Our question is the transport in between. Our first choice is the train, but Koblenz-Caen takes almost a day., with two stops including Paris. Koblenz- Lille is shorter, but longer drive to Bayeaux upon arrival in Lille. We are looking for idea(s) on this topic that we have missed. Better train connections? Bus? Flight?
You have identified the choices fairly accurately. Bayeux (note - no "eaux") is not on a main line, and not that easy to get to except from Paris.
Railways in France are pretty much spokes around Paris - usually the best way to get anywhere by train is through Paris and including changing stations there.
Going via Lille will cost more because of more tolls and the Normandy Bridge which gets into the wallet a little deeper.
Flying won't help.
Either stay the night in Paris or be comfortable with the longish journey.
You will be making connections or transfers, not stops. Stops are what the train does every time it gets to a station, even if you stay on.
As suggested, these two areas don't go together. You have to decide how valuable "slack" time is on your precious vacation.
If America in World War II is your focus, perhaps you should settle for the French "Cider Route" instead of the Mosel wine area. Or start in Alsace's wine route, which is at least in France. There are also French wine regions you could visit that are much closer to Normandy. If I have mistaken your interest in wine, there are also castles and rivers in France.
I'm not saying there's anything wrong with the Mosel. I'm saying that we paired it with the Middle Rhine and Cologne, not with eastern France! Driving from Lille to Normandy without any interest in the stops along the way is poor planning. For example, you are skipping the massive favorite of Brugge, Belgium, not to mention Trier and other high points. I actually prefer Antwerp to Bruges, and BRU is a good place to fly home from instead of Paris.
As our host Rick writes, "assume you'll return."
These connections may be worth considering.
RE 4104
Dep 09:32 TRIER HBF (Germany)
Arr 10:41 SAARBRUECKEN HBF (Germany)
ICE 9556
Dep 11:01 SAARBRUECKEN HBF (Germany)
Arr 12:51 PARIS EST (France)
Transfer from PARIS EST to PARIS ST LAZARE, also with time for lunch.
IC 3311
Dep 15:08 PARIS ST LAZARE (France)
Arr 17:18 BAYEUX (France)
If you like, you can drop the car in Köln. There are train connections to Bayeux or Caen with 1 change in Paris needing about 6 and 7 hours. You can get out in one of the two places depending the rental company you want to use. A much used site for buying tickets and planning train journeys is: https://www.bahn.de/p_en/view/