>- Why did Hulda Clark choose 30Khz as her main frequency
Dr. Clark chose 30 kHz, simply because it passes more current than lower frequencies do. I agree with this as the 30 kHz has a stronger signal at the higher frequencies but this is not the only part to the answer.
>- the lower the frequency gets, the more harder the wave hits you and you can feel it more.
Yes, 2,5 kHz and lower frequencies are felt more strongly and this is a function of nerve and muscle sensitivity. It does however produce results that 30 kHz does not at times.
>- confused as to which frequency is better, a higher frequency, or a lower frequency for the overall body.