A followup: Found Susan's parents thanks to a fellow who posted on another message board, looking for Susan's mother's second husband (William STANLEY) and family. He gave me enough clues to let me add 2 more generations to my tree within about 24 hours.
For other TN Bright researchers: Susan's parents were Daniel BRIGHT and Cynthia/Sintha/Cyntha BAKER, in Knox Cty TN, married Dec. 1, 1859. Daniel fought on the Union side in the Civil War and died in December of (most likely) 1861 at Somerset, KY, undoubtedbly never seeing his new daughter. Cynthia remarried William Stanley in Union Cty in 1870. Cynthia's father, Anderson Baker, was appointed Susan's guardian in 1870 for purposes of collecting a Civil War pension on her behalf, and Susan probably lived with her grandfather's family until she married in 1878.
Cynthia's second husband may have deserted his second family and headed for Missouri (don't know where; that info came from the other post). In the 1880 census Cynthia ("Sintha Stanley") shows up in the Knox Cty household of her daughter Susan and son-in-law, John Russell Bridges (my g-g-grandfather). Still don't know when Cynthia died or where she is buried.
Also tracked Daniel's parents, Elias Rittenhouse BRIGHT and Deborah HAWKINS (or HANKINS).