I'm just so sad that you are going through this. You are doing your best for these birds and really are to be congratulated on that and also investigating further after their death, especially when you have small children to look after. The more knowledge you gain from each case, the better able you are to look after the others and make appropriate decisions.
I know that horrible helpless feeling, but sadly it is not possible to heal every illness and Marek's is one of those where there is no set pattern to it or treatment that will fix it. Each case, responds and/or progresses differently. All you can do is support them whilst they fight it and end it for them when their quality of life is not sustainable.... I have been battling it for 3 years now and I understand your heartache. A few do make miraculous recoveries from attacks, but will be prone to more severe attacks in the future, sometimes months or even a year or more later. There are just no hard and fast rules about it. It is a similar virus to the cold sore virus in humans.... you just don't know who has it until they have an outbreak and what triggers an outbreak in one may not trigger it in another. Unfortunately the means of transmission is much less contained than the direct contact needed with our cold sore virus... dander dust from infected birds is inhaled by others with Marek's. Some will be resistant and not get the disease and some will contract it but may not show symptoms for several months.
Are these other photos of Sassy too. The liver does look an unhealthy colour now that you can see more of it, but from what I can see of the heart and lungs, there is nothing seriously abnormal. I would have expected the lungs to be more bright red than they are, but that may be as a result of your means of euthanasia. It would have been helpful for them to have been removed so that they could be viewed in isolation as there may be tumours on the other side, but nothing obvious. It may just be that she was battling a respiratory infection as a result of a compromised immune system, rather than tumours causing respiratory distress.
I'm not going to suggest you try to examine the sciatic nerves on Dandelion if she doesn't make it because I have not had success in identifying and isolating the nerves myself.... but it can be a key factor in confirming Marek's particularly in those with classic leg paralysis as Sassy and Dandelion were/are exhibiting. I've also found tumours in muscles of some birds with Marek's
Anyway, I am still trying to find the video tutorials on doing a necropsy. When I do find them, don't be put off by the extensive nature of them.... pick and choose which parts are going to be appropriate to your bird from the symptoms it displayed, rather than feel you have to spend hours doing a full professional examination. Personally I don't weigh organs because if it is abnormally large I will recognise that, but of course that comes with experience. If you have butchered a few chickens, you get to know what is abnormal.
I'm sorry I can't give you any definitive cause of death from your necropsy photos but my diagnosis from the symptoms, number of birds displaying them, juvenile age of the birds affected and history of having acquired young birds from an independent breeder, all point very significantly towards Marek's.
What I can say is that you are going through a really bad patch right now, but in my experience it has probably peaked and will level out a bit....hopefully.
