The Nyquist rate tells us in a frequency range of B cycles per second, a maximum of 2B code elements per second can be resolved.
The length of a dit is the fundamental unit of time in Morse code. "PARIS" is commonly used word for determining speed, and it's 50 dits long. So at a speed of 70 words per minute, that's 3500 dits per minute, or 58.33 dits per second. The Nyquist rate requires a frequency bandwidth of at least 29.17 Hz to encode something at that rate. Let's call it 30 Hz for a round number.
Given the lower limit of human hearing is 20 Hz, that would imply the lowest possible range is 20 to 50 Hz. That would imply a sidetone frequency in the middle: 35 Hz, with sidebands arising from the amplitude modulation of the sidetone on and off occupying the 15 Hz above and below that.
So for 70 WPM morse code, the theoretically lowest sidetone frequency which could be audible is 35 Hz. It could be as low as 15 Hz, but that would simply be below the audible range of humans.
70 WPM is pretty fast already, and the biggest limiting factor isn't the speed of the code but rather the limits of human hearing. So to a first order approximation, the answer to your question, "How low a sidetone frequency will allow QRQ CW to be copied?" is "as low as you can hear." Hearing loss, genetics, and the frequency response of the speakers or headphones in use will be more limiting factors than the speed of the code.