That's right. You seem more informed than your vet. Marek's is extremely long lived in soil and in structures, even with attempts at disinfecting.
All of your flock has been exposed if it turns out to be Marek's. There's nothing to be done about that. Some with strong immune systems will survive and develop resistance and live normal lives.
I've lost more than my share of chickens to poisoning. In chicks, it kills very quickly. In adults, it kills but may take a day or more. It doesn't sound like poisoning to me, but I'm not a vet.
Let's let the lab do the necropsy and we will take it from there.
By the way, was this guy you got the bantams from a licensed breeder and did he sell or give you these chicks? Is he in business or just an acquaintance? If he knew his flock was carrying Marek's and is selling chickens, he can be shut down with a phone call to the authorities.