I will see if a colleague of mine has the renault diagram official!! The volt drop will confirm if the relay plate has poor connections, as you suspected. Best option is to follow wiring colour to ecu, relay and connect test probs to the relay pin 85 connection, AT THE ECU, when operated, the relay should use all the voltage, your basically checking the earth line. If less than o.2 volts, use fly leads to connect relay to relay plate and do same things at pin 85 at relay. Supply voltage when relay is not operated, If voltage is high >0.2 volts, there then high resistance in relay plate after relay but before ECU earth point.
I think my autodata is older than yours, hence wiring difference.
HTH Jim