Just an update about the situation. The "infamous" guest who challenges most of her items came back to my register last week after not seeing her for three months. When she started with the challenges, I called my GSTL over and explained the situation, trying not to sound accusatory, by saying something along the lines of "This guest challenges a lot of items, can you help me handle the situation". I tried to be discreet and say it without her hearing it, but she unfortunately heard me. She became upset and felt that I was accusing her of lying and the GSTL took her side and scolded me, BUT after denying one of her price challenges.
I saw her last Monday, and when she passed through another cashier's lane (she passed the lane next to me, perhaps she was trying to avoid me after I called the GSTL the previous time), the cashier just gave in to her demands and changed the prices for her without a disagreement.
If the other GSTL who is diligent is there I will ask them to deal with her, but otherwise, I am starting to feel that just changing the prices to get her out of the store might be the way to go with this. I hate to just give in, but I don't want to be scolded by my GSTL over this. I am holding out hope that the guest won't come to my check lane anymore.
Perhaps I handled the situation inappropriately, and if I did, please let me know and I look forward to your feedback.