Not a full answer this but holding speed will not give you maximum L/D, because at Vl/d you'll have very poor speed stability. You'll probably end up using more fuel by increasing/decreasing TLA all the time. Not to mention the uncomfort of your PAX. In addition, jet engines do not operate very efficiently at low RPM (as at speed for best L/D). Dunno 'bout driftdown speed. Would make sense to set this at/close to Vl/d as this will give you best gliding range in zero wind.. Never seen Vl/d (or Vdmin) mentioned in any AOM for JAR 25 aircraft though...