Thanks for the update on necropsy result. There is no way to prevent internal laying, other than hormonal implants to stop ovulation once it is discovered. It is only a matter of time before infection sets in causing EYP or heart/lung failure from build up of pressure on them from the mass of yolks inside them. Detecting internal laying early can depends on a number of factors. If you have a mixed flock it is sometimes easier to identify when a particular hen has stopped laying than in a single breed flock that lay uniform eggs. Monitoring hens that otherwise look like they should be laying (ie plump red comb.... perhaps visiting nest box and singing egg song without producing an egg) for weight gain and abdominal swelling. Usually, they look healthy and you don't realise there is a problem until you start to see the wide stance, upright penguin walk and find the swelling which of course is often disguised by their feathers. By then, things are pretty well progressed though. I lost a hen a couple of months ago to it, that I noticed had the penguin walk and swelling at least 9 months previous, but her condition improved during the winter when she moulted and was no longer ovulating. As soon as her comb became red and fleshy in spring, the swelling started to increase, clearly as a result of her egg laying system coming back into gear and more yolks being deposited in her abdomen. She didn't get an infection but the pressure on her organs took it's toll.
I don't think they will ever get back to laying eggs properly once they have started internal laying because the weight of the mass of eggs in their belly, pulls things even further out of line, making it more difficult still for the eggs released from the ovary to drop into the oviduct and take their proper course.
The superlorin hormone implants have been extremely useful in treating this problem but at hundreds of pounds/dollars every 6 months or so, it's a very expensive business, which my budget just doesn't run to and of course the mass of yolks that have already accumulated are still at risk of developing an infection.
It could still be that Marek's caused her to suffer EYP. There doesn't necessarily have to be a tumour. Marek's compromises the immune system, so infection is more likely to become an issue in many different parts of the body.