Thanks, Vic, for your thoughts. As an additional comment: hypercalcemia secondary to Vit-D overdosing can in extreme cases lead to calcification of soft tissues, but it is not a frequent observation. A healthy kidney can excrete quite a load of extra calcium (which is why hypercalciuria is a frequent observation in Vit-D overdosing).

Your strategy of getting Vit-D from UV exposed mushrooms is a good one. As far as I know, the FDA even approved the powdered form as a form of supplementation in Vit-D deficiency. And yes, Mg is a required cofactor for activation of Vit-D in the liver and kidney from its inactive form.

A sensible diet should provide all three: Vit-D, Mg, and Ca. If in doubt, an occasional blood check can give the relevant feedback for designing one's diet.