I’ve done as recommended by Ulf and Paul to no effect; only the very first (left-most) fader works in the Mixer, and applies to ALL the instruments.
So I constructed an entirely new project consisting of a mix of 21 parts among various brass, woodwind, and percussion sounds–and a measely 1 bar in length. The 21 instruments (all chosen as Solo Player) were loaded in the two HALion Sonic editors, with no repetitions showing up, and they all play as expected. And all 21 instruments show up–with labels (Trumpet, Clarinet, etc.)–in the Mixer. BUT only the left-most Mixer fader “works” and applies to ALL instruments at once–as before!
Would not this behavior suggest a problem with the Mixer rather than HALion Sonic–or the connection between them? Or since apparently nobody else has reported this problem, does it point to something wrong with my Dorico Pro installation? Or am I just doing something stupid? (I can’t imagine a simpler test than the one I just described.)
P.P.S. Ulf’s first suggestion to me was to select the Mix tab in the HALsion Sonic editer and assign each instrument to a specific channel (Main, 1, 2, … 15). This does work, sort of. First, the Mixer doesn’t show which channel is applied to each instrument-labelled fader, so some trial and error guess work is required; second, this only allows 16 channel specifications, so in the the second HALsion Sonic editer, there’s no place to go. In any case, it strikes me as ridiculus that we would need to go through that kind of preperation for a function that should be transparent with regard to a single source of instrument samples (as in another music program I could mention).