That sounds like typical Marek's findings in my experience. I probably would still have eaten him after cutting out the tumour but you have to do what you think is right. I would expect that thorough cooking will kill any fungal infections as well as bacterial.
I've seen muscular tumours as well as visceral ones. The muscular ones were like sausage meat and cooked egg all mixed together so kind of pink and yellow inside when you cut into them. I think you are right that he would probably have died suddenly from the growth on his heart .
Fat in chickens is usually very yellow, so if you are seeing something more off white, it is probably tumour. If you took any photos, please feel free to post them. We can all learn something from these situations. You may need to amend the thread heading with a "Graphic photo" warning though.