PROBLEM SOLVED -- ALMOST. PROGRAM BUG DISCOVERED!
Here's the story: My tempo changes were created in the BB F5 dialog as =percentages= rather than absolute numbers. The BB tracks interpreted those fine but the melody track was totally random. Sometimes it figured it out, sometimes not. Once I went back and re-entered the absolute tempo numbers everything worked fine...

BUT... When I import this corrected file into RealBand and generate the parts the tempos are whacked out again, in strange new ways. Examples: all the band tracks stop dead 10 bars from the end while the melody track makes it to the end; the bar counter on top of the screen is out of sync with the tune; and if I enable the RealBand Drums, all hell breaks loose!

Geez.