I also scare easily, and a mirror wouldn't have helped me at all because I wouldn't be looking into it in time for the anticipated warning. I just talked to everyone in the vicinity and asked them to knock on the hard bit at the doorway to my cubicle, and lo and behold, they usually asked me to do the same! Even the ones who were a little harder to train just got the knocking treatment from me, and they eventually picked it up.
But if the mirror works for you, don't let her bully you. If you feel you've ruffled her feathers, that's sort of her problem. Talking to HR may help, but only if you feel it's a huge problem. Otherwise, it doesn't seem that big a deal.
... that said, if she does pick her nose at work or look at porn, enjoy your blackmail fodder!
