You couldn't have given a more "right" answer! Good job all the way. I may be able to help you with sexing Fuzzy if you can attach a clear, color photo focusing on her upper body and face.
Now, the unfortunate news is that 'tiels are awfully prone to tumors, liver and kidney disease. There are a few schools of thought on 'why' and quite often right at the top of the list is diet.
You are doing a remarkable job in that area so unless the human food includes any red meat or processed foods which usually have too much salt and who knows what in chemicals - I'd say let's rule that causation out.
It leaves us with internal tumors, probably pressing on nerves in the spine and leading to what you're seeing. It's like a human who has a bad nerve in their upper back/spine with resultant inability to use a leg or even pain in the feet or anywhere as remote and unbelievable as it gets given the nerve compression is in the upper back.
Disease of liver or kidney will sometimes do this as well and again it's a matter of nerves.
Depending on what is found, there are treatments available to either cure or regress both disease or tumor. Sometimes though, the only option is make Fuzzy comfortable, treat the symptoms as they arise and love every day remaining
There is some belief in the avian science world that these birds are prone to such negative things because they've been overbred and DNA indicates that the undesirable genes carrying the weaknesses and problems have been paired up enough in all of this breeding to make the problems more dominant and often seen.
So, for now, let's address the symptom:
Lower her perches so her tail is just off the ground when she stands straight on it (perch). I suggest one of those metal platforms that hook on the inside of the cage . You can fold over a NON terrycloth kitchen towel on the platform to give her some cushioning in her legs and feet. It will be like standing on one of those thickly padded kitchen or restaurant floor pads we see a lot of these days.
You can also get a rope perch to install into her cage, still keeping it low however. Choose one that's got more circumference that a 'tiel generally needs. You want to keep her feet almost flat, but still needing to curl a bit for grip. These perches are also like giving birds a mattress and eases the stress on their legs and toes.
Finally, let's try getting in some all natural sweet potatoes and Pedialyte; both from the human baby aisle. Be sure the sweet potato is nothing but sweet potato and water for processing in the jar. Nothing else.
A boost of beta carotene will work if she's deficient and perhaps solve the problem all by itself. In any case, it will not harm her and it's a really great food for ALL companion birds
The Pedialyte (plain, natural) will boost electrolytes and that's also a good thing. I'd give her one bowl of the electrolyte once a day and after about 6 hrs switch back to plain water. It's ok to do this for a few days to see if it makes any difference.
Keep me informed as to her hands on visits and chemistries. I'm here to support you both.
And remember to try attaching the photo.
Keep up the good work with her