When would a testable double check valve be adequate protection in place of a RPZ?

An RP is for high hazard and a double check is for low hazard. Low hazard protection can never replace a high hazard application.

An RP is effective against backpressure, backflow, and backsiphonage and may be used to isolate health or non-health hazards.

A DC is effective against backpressure, backflow, and backsiphonage but should be used to isolate only non-health hazards.