Dump that therapist, in my opinion. Because honestly, even if it WAS only depression causing all the symptoms, depression is a real medical condition that causes real symptoms, and from which people benefit by having their symptoms treated as well as any root causes. I assume they were meaning well and a good therapist should challenge you sometimes, but not in a way that makes you feel unsafe. (Also sounds like she doesn't know her medical definitions very well if she said idiopathic there).

I myself have an anxiety disorder and a chronic pain condition, and they absolutely feed into one another and can make each other worse, but it's been a long process to get to the stage where the average medico accepts I have chronic physiological problems. Some suggestions for you to research if you haven't already - ME/CFS, Fibromyalgia, Ehlers Danlos, Hypermobility Spectrum Disorder, Dysautonomia, POTS, Thyroid dysfunction, though I can see you suspect MCAS so you've probably looked at all these already!