I'm really sorry to read this. You've had unbelievable bad luck, most of us don't have it anywhere this bad no matter where we got the chicks.
I don't know if all these symptoms would be caused by lymphoid leukosis but with this virus you did nothing wrong. There is absolutely nothing you could have done. It hurts but you have no reason to feel guilty.
I'm no medical expert and am sure not going to argue with a professional. From reading, lymphoid leukosis is a virus that can be spread either from the mother through the hatching egg or from other chickens. Really young chicks are more susceptible to symptoms compared to older chickens. One source said it could be spread through the Marek's vaccination. Sometimes they are contaminated.The first year went perfectly; no illness, injury, loss. At around a year old one of our hens started laying only soft shell. Then we had a series of undetermined illnesses, just lethargy and weakness. Took them to the vet and bloodwork showed toxic changes but no answers. Then we lost one suddenly. Necropsy confirmed lymphoid leukosis, likely from the hatchery.
MyPetChicken is not a hatchery. They traditionally get their chicks from different hatcheries. Even if they were all from the same hatchery I would not expect different breeds to pass this down through the eggs. I think it more likely they were exposed as baby chicks.I should give a disclaimer that I’m not sharing this as definitive evidence that it came from the hatchery, but my vet suggested this is the likely culprit based on timelines and presentation. Our first 5 are from MPC, and we have a variety of breeds.
I don't know if all these symptoms would be caused by lymphoid leukosis but with this virus you did nothing wrong. There is absolutely nothing you could have done. It hurts but you have no reason to feel guilty.