Your chickens pcr test was negative in the past, so it is hard to know if they had MG/MS or something else. I would use caution selling eggs for hatching, and baby chicks just in case.
Actually, I am not sure. They probably would test positive with a necropsy and tissue samples after death, but maybe not if they just had a pcr test if they were not having symptoms. My microbiology friend has always said that Mycoplasma can be hard to culture, even in humans. Your state vet lab...