Can you get a better photo of the lump? Not sure what that is.but it is infested with mites too. But its a different kind of mite than the scaly ones i think?
You mention a different kind of mite than SLM, do you see them in the housing? Can you get a Permethrin based poultry dust or spray to treat the birds and housing? That may help.
I think I remember you said you had Ivermectin. This can help treat mites/lice on the birds, but if the bugs are still in the coop/housing, then the birds will continue to get re-infested.
For some it seems to be a never ending battle with bugs. Hopefully you can find a solution.
I do agree that trying oil may be helpful and soothing to his legs.