there is a command that 'unfolds' the choruses to one long file so no need to 'spell out', and often its easier to put the changes in where you can see them rather than mess around with bar numbers and chorus numbers which I find leads me to confusion.

when you create the backing in BIAB does the file play correctly before you export? knowing that would help work out where the error is occurring.

if it plays fine in BIAB the problem is happening when you export. sadly with the Mac version you don't get RealBand which opens BIAB files and is dead easy to record in - just as easy as Garageband.

first of all, please let us know if the BIAB track plays perfectly before you start any exports

if it does play properly, maybe taking this thread to the BIAB Mac forum might help you with replies from other Mac users who use Garageband and export BIAB files to that program.

You could also ask whether 'the mix-to-MP3 option does not work on the Mac' is true for other Mac users or just something affecting you.