It is not true that you have to print the ticket; we bought tickets in November and just showed the bar code on our phone at the entrances to the wings -- no issues at all; they do a pass sanitaire check at the security line. We had no real line at the Carousel in November for security.
Third party sites offer zero advantages and there is no such thing as a 'skip the line ticket' and third party sites have no special access to extra special tickets that offer special benefits. It is all hype for the gullible. Just buy tickets on official web sites and show them on your phone -- for pretty much every entry in Paris from the Carnavalet which is free but requires a reservation to the Opera. We didn't have to print a single ticket this fall. All the venues we visited had timed tickets we bought on line. You do need to book them a few days in advance in some cases.
The Orsay usually is a museum I would not approach without ticket ahead where you use a different entry door but I did notice this fall that the main ticket line was not very long - which is a real change from the before times. So apparently you can show up there and buy a ticket. I don't know about the Louvre -- we had to show our ticket as well as our Pass Sanitaire to get into the pyramid just to prove we had tickets; in the past you could buy them under the pyramid once you entered.