Can someone make sense of Sota"s ZHC5 frequencies???

I am planning you buy a zapper that my 90 year old mother can use without risking burn marks. I came up with the Sota ZHC5 which I think has only wrist straps, no direct metal contact. Before deciding, I am looking for answers for these questions.
1) Has anyone here experience with this model and can verify that it won't have a chance to leave burn marks?
2) How satisfied are you with it? Why?
3) How do you work with the frequencies? There are options of 30,000Hz, 2,500Hz, 1,000 Hz, and 725 Hz. Looking at Hulda Clark 's book, she mentions many many frequencies in between these above, so how can these for frequencies above be of any real use. When I called Sota they said, "just experiment", and gave me a web site which tells me nothing I understand.
4) Would you recommend another zapper that is burn proof?
I would greatly appreciate hearing from some of you.
Thanks.