What does chicken jail look like? Is it elevated with a wire mesh floor so her belly gets cooled? A photo might be interesting. Anyway, I leave mine in for 72 hours , not 48, and if it doesn't work I keep trying. Some can be a lot harder than others to break.
I've read that myth before, and that is pure myth. I almost never give my broody hens eggs until they have been broody for a week to make sure they are seriously broody and to collect hatching eggs and I've never had one of them quit. Three weeks is pure myth.
It's not as bad as you think yet. Before a hen even starts to lay she stores up excess fat. People that wind up butchering their hens are often amazed at how much fat this can be. This fat is mostly what she lives off of while broody so she can stay on the nest and take care of the eggs instead of having to go out and find food and water all the time. She probably still has some reserves at 5 weeks. She will lose weight but it's fat put there for that purpose. No big deal.
Most hens will break from being broody when they use up that fat. Most doesn't necessarily mean all. I'd go back to the broody jail for longer periods of time. Make sure she had food and water but nothing like a nest. I have had a few that took a lot of days to break.
