Does this Schottky provide MOSFET transient protection?
I don't see any useful functionality for that part. In the Jameco schematic the IRF46 MOSFET used will handle very large transient currents through the body diode, which is directly in parallel. It also has a reasonably healthy 28A/11mJ avalanche rating. 

A reverse connection of the load will put all the source voltage (minus a diode drop) across the sense resistor. Chances are that will damage the resistors before anything else. 

A series Schottky might be more useful, as might a TVS or other overvoltage protection (the low voltage Schottky will break down but that's hardly reliable).

Speculation as to how this ended up here- copying of some bridge switching configuration or perhaps they had problems with MOSFETs dying and upgraded the MOSFET and added the diode blindly. Some MOSFETs don't do well in linear mode, datasheet SOA notwithstanding.