Hi Bantam Girl,
We are very sorry to hear about the loss of your hen, it does sound by what you describe from your final vet visit that she was suffering from a nasty infection in her leg.
We also apologise, it seems that we read incorrectly that your hen was ½ a year old instead of 1 ½ years old. Mareks is usually developed between of 16 – 20 weeks of age, if your bird had been that age and was displaying these symptoms it could have been a likely cause.
Infections in feet and legs of hens can be caused by a number of reasons, it is quite common an initial, not very obvious skin wound has allowed bacteria to enter and cause infection. This can come about by a simple scratch to the skin which becomes infected. Check your perches are not too high, as when birds fly down in the mornings they can bang hard into the floor and if shavings are sharp this can be a cause of damage to feet. Check your perches have no rough edges as again splinters can introduce infection and check your run area generally for sharp objects. Obviously none of these may have been the actual cause but they are always things worth bearing in mind when keeping hens.