During my 2012 visit to the Louvre I tested four possible entrances (your results may be different “:-)”. 1) The Pyramid, 2) Passage Richelieu, 3) Galerie du Carrousel, 4) Port de Lion. My personal favorite but perhaps not (yet?) wheelchair accessible is the Porte des Lions. I **discovered** this entrance during my “no other destination” visit. My hotel was within easy walking distance and I past by this entrance before reaching the Pyramid. This entrance was/(is?) not heavily used, and you’ll be much closer to the Mona Lisa gallery. The bus lines with stops truly <<à quelque pas>> from the Portes des Lions entrance are 24, 27, 69, and 72.
p.s. I noticed on the Louvre website that you can also gain access from the Cour Carrée.