What you need to be aware of with abscesses in chickens is that the puss goes hard like cheese, so you will not normally be able to draw it out with a needle or burst it like a normal spot or abscess. If that is an abscess, I would clip away the surrounding feathers and hot compress it with Epsom salts poultices and get it as cleaned up as you can. Unfortunately surgical removal may be the only option, but if a vet cannot tell if it is an abscess or tumour, I can understand your reticence to cut into it. In that location, it could be an old injury from a cockerel that has got infected, but the lameness is worrying as regards Marek's especially with the previous bird's problems. How old was the hen that died? I've had a bird with Marek's that had a subcutaneous tumour and it was just one large one on the leg, so it can happen.
I agree that the vet you saw does not understand the new legislation. The only way you are supposed to access antibiotics for chickens is via a script from the vet.
Emily will be feeling better soon! 
