Thanks for having investigated on your side!
It seems you took a different approach than mine right a the beginning, by choosing individual players instead of creating first a string divisi, which leads to the a2, a4 problem you’re pointing.
On my side, condensing divisi works well, beside two things : 1) glissandi line that disappear 2) chords and artificial harmonics that stack instead of being condensed when I use manual condensing.
So it seems there’s a bug with manual condensing that don’t take into account preferences regarding condensing unisons. If that bug was fixed, I think everything would work fine (beside gliss .line disappearing, which is another problem that was pointed out in 2019 I think Glissando is lost during condensing)
Regarding individual part, I will create one part per desk, which is quite easy and convenient (both for the engraver and the performer). There won’t be multiple staves in these parts.