Should Factory Reset return the phone to the original version Android installed on it?
Factory reset only delete user data (userdata partition) and restore settings to the  default values, meaning every settings will be restored to the factory state. This doesn't apply to the system, (firmware/ROM)  which remains untouched. 

By original factory ROM, do you mean the one the phone came with? This can be done by downgrading the phone firmware: flashing lower version firmware (with a lower android version). 

Not all device support downgrading though,  since earlier versions can have bugs, security holes, etc... Many device makers such as HTC, Huawei, Motorola (Lenovo)... don't allow downgrading.