well your only two days for the Musee d'Orsay are Tuesdays which is generally the worst day for crowds and lines. Get timed tickets and early probably rather than later. You then join the ticket line for those with your reservation slot -- they are by half hour and the lines are to the right as you face the museum. Do not check coats; those lines are horrendous on days the museum is crowded. I often carry a net bag or nylon bag I can use to cram my coat or jacket in a hot museum without efficient coat check. (the Louvre you just go find a coat locker; the Orsay you have to stand in a line that can easily be half an hour or more long.). I would guess the 19th would be better than April 2 but that is just a guess --
The Marmottan is wonderful and in my experience not terribly crowded. The Orangerie of course has Monet's famous water lily murals.
If you don't mind spending a lot of money, you could buy Carte Blanche membership for the Orsay which allows one guest and puts you at the head of lines both for special exhibits and entry. It has its own special line. (I have done this several times as a guest of a French friend who has the card). This entry card also gives you priority at the Orangerie although it is not as dramatic as the Orsay. (you do get to enter the line from the side about 15 persons back from entry.).