Her eyes look clear and react to the light, so I'm not suspecting Mareck's.
Just an FYI, as Parront stated Mareks *sometimes* affects the eyes.
There are numerous Marek's strains. Only some affect the eyes. Others cause paralysis (the classic one leg forward and one leg back as the sciatic nerve is affected). Other strains cause a slow wasting and weakness.
Even vaccinated birds can be affected as the vaccination is "leaky" meaning it only prevents tumor growth from the main strains. The vaccine does not prevent a bird from catching the virus.
During times of stress, such as point of lay, the bird's immune system can be overwhelmed causing tumor growths. Where the tumors grow creates the different symptoms....be it eye, feather follicle, nerve system, or internal organs.
I wouldn't rule out Marek's yet. I hope it is nutritional, but since you are already feeding a good feed with oyster shell, it may not be that. It could be just this bird's nutritional status. Favorelle tend to be on the lowest rung of the pecking order, so she may not have been getting enough nutrition to sustain egg production. It could be muscular skeletal issues of a weak pelvis, an old injury acquired during hatch if bones fused incorrectly or genetic weaknesses.
Without thorough examination and labs, there is no way of really knowing what is causing her issues. All we can do is suggest according to the description of symptoms and hope for the best with you.
I'm glad she seems to be doing better, and hopefully she will rally and do well. I would recommend keeping a careful eye on her overall to make sure she is able to get enough nutrition. Favorelles often don't do well in mixed flocks due to their compliant natures. They are easy for other birds to pick on.
LofMc