Trying to render either the Melody track or the whole mix down, doesn't record the Harmonist I have switched on.
Is there any way - like in Embellish Melody - to make it permanent into the track either as midi info or a rendered wav file?

If you use Harmony menu | 'Convert harmony to melody track', this will write it to the Melody track, and it will be included in the rendered file. You may want to save a copy of the song prior to doing this.
PG Music Inc.