Greg,
As far as your first question, the NEC requires a main bonding jumper at the service disconnect. In a small panel the green screw is the main bonding jumper.
As far as the second question, an EGC (equipment grounding conductor) is required to be installed to the subpanel. This is the only way a ground fault can be cleared. Grounding electrode systems will rarely carry enough current to cause the OCPD to open. The NEC permits the metallic conduit to be the required EGC.
Don(resqcapt19)
