If you have concerns about the screening practices of a Mistress, as upsetting as it may seem, politely tell her you changed your mind and move on. From what you describe, she may have been stalked at some point or a friend/colleague was. Mistresses who read your post would likely have a far more accurate view than this old sub.
Looking back on ancient times (the '80s & '90s) I don't see how independent Mistresses who either rented space at a dungeon or did sessions out of their own apartments or rented spaces essentially gambled that they could figure our with a phone call or two if a prospective sub was legitimate, cop or axe murderer. At least today Mistresses can vet us a lot better, and vice versa.