I wondered about primary hyperparathyroidism too. That and tumors account for 90% of cases of hypercalcemia in humans.
I can’t find any literature on primary hyperparathyroidism in reptiles, only secondary dietary hyperparathyroidism (MBD) and secondary renal hyperparathyroidism due to kidney dysfunction.
hypervitaminosis D can also cause hypercalcemia
looking at the Cham you have a few noticeable signs/symptoms
Hypercalcemia. Which could be related to supplements, MBD, or renal dysfunction (or something more rare)
Cask swelling. I think obesity is a reasonable cause given feeding regimen. As you mentioned, this could lead to multiple organ dysfunction. Obesity is one of the most dangerous diseases on the planet contributing to a huge variety of diseases and mortality
Deformation in the cask. While there may be other causes, MBD seems the most likely culprit
I think you need to look over husbandry in general. Supplementation needs to be dialed in. I don’t see a reason at this point to go away from a standard of calcium no D3 most feedings and a multivitamin with D3 a couple times a month, and don’t think giving more dietary D3 is a good idea since that could lead to hypervitaminosis D and more hypercalcemia. Definitely need to reduce feeding to hopefully get obesity under control. Check on lights/heat for to ensure proper endogenous D3 production. Last, take a good look at the hydration routine and status of the Cham. Dehydration could lead to acute or chronic renal failure
hope some of that helps