The next thing I'd try is to vent the EGR valve to atmosphere (bad Sixto; bad, bad Sixto) or back into the exhaust tract. You might experiment venting to atmosphere very very briefly to see if that fools the sensor. Don't forget to plug the EGR valve flange on the mixing pipe.
You could also try to plug the exhaust end of the EGR valve while keeping the mixing pipe end open. It's a long shot, but since you don't know exaxtly what the sensor is looking for, it's worth a shot.
If you bypass the overboost protection circuit, you can attach a pressure relief valve to the flange in the mixing pipe. There are numerous after market relief valves for turbo applications (Mitsubishi, Audi, VW). You'll want the type that's adjustable so you can dial it in to the stock overboost level. It won't provide the backup of cutting fuel, but it's better than nothing.
I'm not a fan of a solution that doesn't address the engine fault light.