Alias frequencies of the input signal occur at +/-N*fs, as already explained. Your calculations in post #3 are all wrong. Alias frequencies can be negative, e.g. 720 - 800 = -80 MHz, but in a simple sampler, -80 MHz appears as +80 MHz. You need a homodyne detector (I/Q sampler) to separate positive and negative frequency components.
Respectively I would answer the initial question differently. There are no negative harmonics, but negative alias frequencies which can't be separated from positive frequencies by your system.
You may want to review the aliasing and folding chapter of a signal processing text book.