Just did first run through with harpist and they asked for the trem to be written as in image (I drew it in by hand). Not sure how to do this in Dorico, or if its possible. Tried on a different layer but it changed the enharmonic spelling on both.
Both tremolo forms can be found in the repeat section.
For the bisbigliando one enter two dotted quaver/8th notes select both and select Three Strokes Multi-note Tremolo
I was also wondering how to show the auxiliary note. I came up with a trill hiding the trill mark. Intervals can be negative and there is even a Auxiliary note offset in Engraving Mode for horizontal fine tuning.
However the parentheses can’t be hidden as far as I know.