So....

Last night spent 2-3 hours changing things in BIAB in an attempt to correct loading a melody instrument over the one I chose. I solved the problem in that my chosen melody instrument is now retained by going into the "global song override" menu that you get to from the "Song settings" menu (Ctrl-N) and unclicking "Allow loading of any these settings" (sic). If that is checked, the guitar is loaded irrespective of what I select for a melody instrument.

I have sent jazzmandan the file for uploading.