I think you are confused by negative frequencies and what they mean so let me add this explanation.
When you see a spectrum that contains "positive" and "negative" frequencies, each of the frequencies are of the form:
Where $\omega$ is the frequency (in this case angular frequency as $2\pi f$ with f being the frequency in Hz.
Yes you are right.
Aliasing only happens when you sample (either analog to digital conversion or during digital downsampling) a signal and refers to the overlap of signal spectrum due to an inadequate sampling rate; sampling rate below the Nyquist rate.
Aliasing cannot happen at the output of a DAC (digital to analog converter). What happens is, if you do ...