How to reduce synthesis artifacts produced by phase vocoder?
The second artifact can be caused by the fact that phase vocoder slow-down resynthesis can reduce the frequency of any natural tremolo and vibrato in the human speaking voice, as well as any tiny period-to-period jitter in glottal closures.

One possible solution might be to measure the frequencies of tremolo and vibrato, and the jitter statistics, of voiced windows, and re-add those artifacts to (or modulate) the result of the PV resynthesis, if they are absent or too small.