Can AC harmonic quantities be expressed as a sum of phasors of different frequencies without confusion?
Yes, that is fine. As long as you clearly label your different harmonic phasors so you don’t mix them up.
It is nothing more than a convenient short-hand description of the original harmonic content.
That is what I am doing in this example where I calculate the harmonic content of a transformer inrush current.  I actually re-constructed the original waveform from the phasor data at the end (as a check).
Here was the inrush waveform (about 18 cycles worth at 60Hz):

I did a sliding DFT across it so that at each sample point I computed the phasors for harmonics from fundamental up to Nyquist.  Then, to make sure my approach was correct I took those phasors, and at each point, reconstructed the original waveform (below):

Remember that with my approach I am calculating a set of N/2 phasors (N was my sample rate) at each sample point along the waveform - a sliding window for the dft.