I question your use of the term "back Fire". This is usually a sneeze into the carboretor caused by switched spark plug wires or a lean mixture. Firing with the ports open (switched wires) would not strain the starter. However if the safety interlock were compromised, it would allow over advancing the ignition which would fire the ignition early and try to run the outboard backwards. This would put enough force on the bendex drive to shear off the cotter pin or break the starter motor mount. Lucky the cotter pin went and not the motor mount. . . 😮