Vatican museums-how late can you stay and can you backtrack?

You can backtrack but you will be bucking the flow of traffic.
You will not be hurried along except by the flow of traffic.

The Museums offer later evening times, and afternoons seem to generally be less busy than mornings. I don’t recall whether the hours vary by day/night of the week, but their Website should have the latest details on hours and visiting times.

As Bob said, you can backtrack, but crowds could make that difficult. Again, afternoons would likely make it an easier effort. Sometimes certain. rooms get closed, and going back later, they might be opened again, so it could be a good strategy in some cases. The closer you get to the end of the usual flow of traffic, near the Sistine Chapel, space is limited and crowds get packed, especially in stairways. Going backwards could be extremely difficult.

I was just there in October. In some of the more crowded areas, there is a specific route they want you to take, and it is enforced with waist-high barriers. This can be somewhat annoying if the room you want to see is right next to you, but you have to first take 15 or so minutes along a very long and very crowded path before you can wend your way back to it.

However, if one is reasonably agile, and the friendly guard is looking the other way, it might be possible to make one's way over the barrier and see it anyway without taking the long route.

I've been to the Vatican Museums a few times. This past October was by far the most crowded I have ever seen (I visited in the afternoon), with many routes every bit as crowded as the paths exiting a sports stadium. It's a great museum, but the crowds are probably worse than any other museum I have visited, so be warned.