Ask the breeder what the NPIP testing is "required" for qualification in your state or your state should have that listed. Most NPIP do not require testing for MG/MS, it's usually Pullorum, Salmonella and Avian Influenza.
I agree, I would not use that breeder anymore. It does sound like...
O.k. I see you added more photos since I was last on, these are helpful.
Sadly, I'd lean more towards this being a leg bone deformity. Valgus-Varus, with this being Varus deformation.
Usually this is not correctable, and it can be genetic. Sometimes parent stock nutrition deficiency can be...
What are you feeding them?
What's the Tylan50 for? You suspect MS/MG? Any respiratory symptoms in your flock? Any swelling of the joints like fluid or oozing?
If you need it, Tylan50 dose if 0.25ml per pound of weight given orally 3 times a day for 5 days in a row.
3.5lb for a 13wk old BCM...