I first want to say I fully support your wishes but in regard to your second wish,
"It would also be nice to be able to copy the whole top note line of e.g. the “real horn lines” to the soloist track in one go, in order to be able to “thicken” the lines by e.g. the flute."
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According to what year release version one has of BIAB, RealTrack "thicken" feature has been an included feature for the last several years. My year version is 2021 and I confirmed it's available for all the Legacy Channels including the Drums, Soloist and Melody Channels by two methods.

1. Rt click in the description area of a channel, select Track Settings, Duplicate RealTracks (To Thicken Sound....).
A range of 0 to 10 duplicate RealTracks can be added to thicken the sound.

2. Alternately, open up the RealTracks Picker, select the Medley Button to add and edit duplicate tracks to thicken the sound.

BIAB 2025:RB 2025, Latest builds: Dell Optiplex 7040 Desktop; Windows-10-64 bit, Intel Core i7-6700 3.4GHz CPU and 16 GB Ram Memory.