If I rent a car in Caen, may I return it to the airport CDG on a monday morning?
Certainly. If you're staying close to CDG Sunday night... That's a long drive from Caen. When is your flight?
You mention trains in the post title - there are no direct trains from Caen to CDG and you will need to use at least two Metro/RER lines to get there from Saint Lazare, the main Paris station for that area. Somebody else asked the same question in the reverse direction CDG-Caen a few weeks ago and the general opinion from people who know French roads was that driving would be faster and more convenient than rail.
I am grateful for your replies regarding getting to Caen by train, and will rent a car at the airport for the journey north to Caen.