You didn't say which version of BIAB you are using (I ask since you say it's been awhile).

That being said, when you open a MIDI file with BIAB, the entire MIDI file gets loaded into the Melody track. It does not split out the MIDI to the various BIAB tracks (and other than the drums, how could it, and what if there were more MIDI tracks than BIAB tracks). Besides the BIAB tracks are populated based on styles, not input MIDI files.

Now that I've said all that, what you can do (if your version of BIAB supports this) is use the Sequencer feature for your multi-channel MIDI. Select a MIDI channel and move it to the soloist channel. You can then use the "Copy/Move Track" feature to then move the Soloist track to any of the other tracks (bass, for example), and then freeze the bass track. What will now play is the MIDI performance from the track you imported, not a generated part based on a BIAB style. You can then continue doing this for the other tracks. But remember, if you unfreeze a track and regenerate, it will revert back to the style. Also, while frozen, if you change the chord progression and regenerate, nothing will change on a frozen track.

To display multiple BIAB tracks in the notation view, see the "Selecting Parts In The Lead Sheet" section of the user manual (page 274 in the BIAB 2015 user manual). You basically CTRL-click the various tracks you want displayed.

Can't answer #3, and for #4, yes, you will need to purchase either a good VSTi or use a hardware sampler. Which one to get is a matter of taste, and you will probably not find exactly what you are looking for, but anything is better than the default MIDI sounds you get with Windows. It can get expensive to get great brass sound.

John

Laptop-HP Omen I7 Win11Pro 32GB 2x2TB, 1x4TB SSD
Desktop-ASUS-I7 Win10Pro 32GB 2x1.5TB, 2x2TB, 1x4TB SATA

BB2025/UMC404HD/Casios/Cakewalk/Reaper/Studio One/MixBus/Notion/Finale/Dorico/Noteworthy/NI/Halion/IK

http://www.sus4chord.com