[Richard Farrant]

Originally Posted by motto

I understand the issues now with the nuts being captive or inaccessible you only have the head of the screw to work on.
The most effective tool I encountered for undoing screws on aircraft (sometimes by the hundred) was a rivet gun with tip adaptor and handle. The principle was that the shock load was applied completely independent of any radial force unlike an impact driver. Of course the screws in that case were 3/16" or 1/4" diameter.
To translate the principle to what you are attempting would require a jack-hammer or pavement breaker fitted with an appropriate tip and a handle to apply rotational force to it.
Maybe just fantasy but who knows what people have laying about the place.

David

David,
You have given me an idea here, an air needle gun or descaling tool. Remove the needles and make an adaptor to fit on of the screwdriver ends in to.

I have gone through the pain of these countersink screws on wartime British armour although they have deeper heads the slot is shallow and narrow. The head being more pronounced allows a flat drift to be used to knock the head sideways a fraction to break rust seal and allow penetrating fluid in.