Corn is a big energy source, making a lot of horses 'hot.' I would be careful about the amount of weight you put on her and how quickly. Being over weight or having too rapid weight gain could make problems worse for her lameness.
I would try careful deworming, free range hay, increasing feed slightly as time progresses, (sweet feed does add weight to horses not being worked) and using canola oil. Like you said, I'd also get the vet out soon. I'm sure you'd get much more reliable advice. Good luck!