Well at least it confirms that you did the right thing in having her put to sleep. I hope that is some comfort to you. I'm no expert but the way I read it, she was riddled with tumours in lungs liver and spleen as well as that mass on her back and her breast muscle tissue was also turning necrotic. Poor girl!
I'm not sure they can tell the difference between Marek's and Avian Leukosis without further expensive testing and I don't think you would gain much in knowing which it is.
I have no experience with Avian Leukosis but two birds exhibiting lameness together with one of them having a necropsy report of internal tumours consistent with those 2 diseases would probably indicate it is Marek's. Certainly that is the more common of the two. Sadly the virus has little or no respect for cleanliness, so do not feel it is a reflection on you. Infected dander dust can be carried on the wind or clothes/skin and hair from an infected bird to yours or via an infected bird, which may appear perfectly healthy and come from a respected breeder.
Hopefully your other birds will have some resistance to it, so don't feel that it is necessarily a death sentence for the whole flock. It might be worth using a vacuum cleaner to suck all the finer dust out of the coop as that is the stuff that is likely to become air bourne and infect other birds via inhalation. And then disinfect it after that.
On a positive note, the faecal exam shows no internal parasites (worms) or worm eggs, so that is a good thing.
