I have had Marek's go through my flock a couple of years ago. I only knew that it was because one of the hens that was affected was one of my daughter's favorites. It had trouble walking, flying and getting up on the the perch. We took the hen to our 4H vet and she had to euthanize her because she said the hen had tumors growing on some of her nerves. The most noticable tumor was on her back near her preening gland. This hen was over one year old, so even though you will see on the web that most birds get Marek's at a certain age that is not always the case.
I ordered the vacine from First State Veterinary Supply, but you could order it anywhere online. Then we vacinated all of our flock. I only lost 2 birds out of 30. You can call the guy who owns First State, he is really helpfull. Make sure that it is Marek's disease before you go through all of this though, it is not cheap to get the vaccine. He did send me the correct needles, etc. so that I didn't have to make a seperate trip to the farm store.
Before you go through all of this, do you have a vet that could take a look at your hen? Did you wash off her foot and look for a cut or anything? I saw that you looked for the bumblefoot spot. I just don't want you to have any unecessary expenses. Maybe she jumped off of the perch and tweeked her ankle a bit.
My hens aren't really laying super great right now, nor or alot of other people that I know that have chickens so the lack of eggs might not have anything to do with it, not sure. It is always so tough to tell with chickens because there are so many things that can happen and they really do try to hide alot of their symptoms so they don't get picked on by the rest of the flock.