Bumblefoot usually causes a scab and swelling on the pad of the foot. I am not a vet, and just am a chicken owner just like you. There are several conditions that you can Google, such as MS in chickens, gout which causes swelling of both ankles, and osteopetrosis, which causes hard swelling of the legs, but the feet do not swell.
Yours looks more like a bacterial joint infection, such as MS. Other bacteria that can cause this are E.coli, staphylococcus, and others. MS symptoms can be treated with antibiotics such as Tylan 50 injectable, given orally or as a shot into breast muscle. Dosage is 0.2 ml per pound twice a day for 5 days. If you have a local vet, they may be able to give you a better antibiotic that would be more broad spectrum, and treat more bacteria than mycoplasma. The vet also could help in diagnosis. Sorry you are dealing with this.
