As someone who has had Marek's in their flock for several years I am probably more experienced at recognising the symptoms and how it exhibits than much of what you read. Once birds get it, they have it for life and are carriers. It is like the Herpes virus in humans. You can't tell who has it and who hasn't until they have an outbreak. Some birds die from the initial outbreak, some make miraculous recoveries but will almost certainly have further and usually more serious attacks, weeks months or even years later but have a good quality of life in between.
My view is that once there is an outbreak of it, your flock has already been exposed, and if you wish to care for the sick bird rather than cull them, they will do best if they have the company of other chickens. Stress makes Marek's worse. Keeping them as happy and comfortable as possible, plenty of sunlight and access to grass and the company of other chickens but within a safe environment have been the factors that have shown the most benefit for my Marek's birds, but some do still die of course.
My view is that once there is an outbreak of it, your flock has already been exposed, and if you wish to care for the sick bird rather than cull them, they will do best if they have the company of other chickens. Stress makes Marek's worse. Keeping them as happy and comfortable as possible, plenty of sunlight and access to grass and the company of other chickens but within a safe environment have been the factors that have shown the most benefit for my Marek's birds, but some do still die of course.