Three approaches (and thank you for a clear explanation of your requirements ).
1. Black Smooth leather messenger bag....must be light weight and used without a bulky insert . Width is most important ..it needs to be thin and tall not (short and square like most small bags ) . The leather should be polished looking . Bag should be in that 3-4 wide size . This is important so bag collapses into your body . You have to look close at the leather many popular bags are industrial strength ...designed for rough use . I will look for a couple of examples . (Artisan and Artist used to have bags like this , Oberwerth of course is great , I have thin black Billingham that is coated canvas that would work (its all black ) . Black Nylon also works so Prada makes a formal looking bag .
2. Lens only bag ....assume you have the camera and the 50MM on your shoulder (black smooth leather strap ) . You would need a very small black leather bag for two lenses ,a few extra batteries . This is the approach I would use and I have one of Overgaard s small MONO messenger bags (way way too expensive ).
3. Get a Leica Q2 MONO and forget the bag ....this would be much more useful and elegant solution . Plus you would have 28/35/50mm FOV at a touch with plenty of pixels .
Considerations
Based on your stated requirements ...you don t need the bulk of a true working bag . Ditch the insert and use a camera wrap or neoprene pouch . Or just stuff that white silk scarf you wore with your tuxedo ! into the bag as padding .
Personally I favor the lens only bag because I want the camera out all the time .
A Pro would have a small flash and this could be tricking in the Louve.
When I am stuck I bribe my wife (very expensive choice ) to use a purse that can hold a lens .
whglobalpack.com
I am really jealous of your getting to photograph at the Louve . I was scheduled to be in Paris the entire month of June but a last minute family issue required a postponement . I know this assignment well and you have the 3 focal lengths I would want .