You answered your own question - BIAB is not a DAW and will not combine audio and midi like a DAW. You need to rethink what this program does. When you said:

"... starting a new song, setting the key and tempo, and this time importing the midi file (this resulted in the right chords appearing) and then -- before even bothering with the wav version or the Melodist or Soloist features -- just picked a RealStyle and pressed play. The style was "CBGUIRO.STY. Classic Country w Guiro(165 RS)". Using the mixer I muted everything but the acoustic guitar. Sounded beautiful."

That is what BIAB does. Now continue on with the Melodist and Soloist features. Get it even sounding better, then export every individual track to a DAW (RB, Sonar, Reaper....) and then insert the original bass wav file on a separate track - mix -master and publish.