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.

Where is the suggested location for the RPZ, before or after the water meter?

For containment purposes, the backflow preventer should always be installed immediately after the water meter. The domestic water should have a valve before & after the water meter, then install the backflow preventer. It's actually nice to have an additional valve after the backflow preventer should the backflow preventer ever need to be replaced.

For isolation purposes, a backflow preventer should be on the water feed (boiler, irrigation, etc) to the potential hazard. For a boiler feed, the backflow preventer should be installed before the pressure reducing valve.

Thanks for asking!