I went this year in the last week of February and thats about as low season as it gets. Only French schools are on holiday that week, and French families go sking for spring half term. The weeks to avoid are when UK and Irish schools are off, which was the week before I went. I saw reports that it was jammed that week.
So I would look at the last week of February, first week of March. Middle of March starts to get busy again. The only thing about this time is shorter park hours, and both ride and restaurant closures.
I did a two night / 3 day trip. The big difference between American Parks and Paris is that the prices listed on the website are hotel and park ticket packages which include breakfast as standard. You get park tickets for your arrival day AND your departure days. I am from Ireland and I got a 7am flight from my country and was in the park for 1pm.
On my departure day, I checked out, put my luggage in the storage room , had a full day at the park, then when the park closed I collected my parks and stayed the night at a hotel in Val d'Europe, which is right beside Disney. I got my flight home the next day.