Disclaimer: The Louvre is one of the most popular museum in the world, so there will be tons of people every day and every hours; just be patient, and if you miss something, don't worry, come back to it next time you are in Paris.
Don't go on Mondays, even if the museum is open, other Museums (municipal museums, like the Musée d'Orsay) in Paris are closed on Mondays so everyone divert to the Louvre.
Try to get there as early as possible (opening) or just at lunch time when tour groups leave for lunch and use alternate entry points (other than the pyramid).
I never went in the evening, but it can be fun (Wednesday and Friday).
I think it is allowed to leave at mid-day for lunch and come back later on (a quick google says it is allowed, but always ask on site, just in case).
Chairs are probably (surely) not allowed; selfie sticks are maybe still allowed, but I would not bring one; there are just too many people to safely use it. (leave it at home). There are exceptions for people who sit down and copy arts, but you need a copyist card from the Louvre).
For the visit, I would leave all non-essential stuff at the the hotel/apartment, that includes backpacks, umbrellas, extra clothing... just bring the minimum with you (ids, money, camera/phone).
There is a cloakroom under the pyramid if you need to stash your bags if you cannot be without them.
Wear comfortable shoes and light attire, it can be hot during the day, there is not much ventilation in the museum.