@Eggcessive , I guess it's possible. When they died, the second rooster seemed to have paralyzed legs and the hen with the puffy face (who didn't have a necropsy) had a strangely bent neck. They didn't do a PCR for Marek's. Only for Newcastle and AI (negative). But the two chickens that underwent necropsy didn't have any internal tumors - the necropsy resluts describe the findings on internal organs. They both had bleeding lesions on their livers, though, and were found to be anemic and malnourished (that must be disease related - I feed my chickens abundantly with shop-bought layer feed, they always leave feed over), other than the egg peritonitis in one and the coccidiosis in the other.
Weird thing, though - of the 6 adult chickens that died, 5 had single combs.Of the 3 survivors, 2 have rose combs and the surviving chicks of the broody hens that died also have rose combs (though the single combed chicks were killed by predators rather than disease, as far as I could see). Don't know if that's significant somehow.