If Peter Smith is right, then dive-bombing was neither a German nor a US invention but a British one!
Sad to say no - an American one and pre WW1!
"American soldier of fortune Leonard Warden Bonney who, flying a Moisant pusher biplane for Mexican Gen. Carranza's government, carried out bombing attacks at Tampico and Vera Cruz against revolutionaries in the Civil War of 1913-15. Taught by the Wright Brothers to fly, Bonney hurled small bombs with shotgun shell detonators developed by Mexican mining engineers in shallow diving attacks. Diving from about 1500-ft, Bonney would aim the Moisant directly at the target without benefit of any sighting mechanism. Dropping the bombs just as he pulled out of his dive, it was not uncommon for the fearless airman to score several direct hits that sent the revolutionaries scurrying and Bonney back to base with dozens of bullet holes testifying to his adventures. Professing to have hit the target about 50% of the time, Bonney became an early proponent of dive-bombing while a Naval flight instructor during WWI. "
The first example of dive bombing in WW1 is British.
"October 1914, Royal Naval Air Service pilot Fit. Lt. R.G. Marix flying a Sopwith Tabloid carried out a daring, most successful surprise dive-bombing attack against the German Zeppelin hangars at Dusseldorf. Dropping 20-lb Cooper bombs with contact fuses out of a foggy dawn sky, Marix' small missiles caused the shed to collapse, destroying Zeppelin L. IX the process."
However another early pioneer was the American pilot Robert L. Rockwell a member of the Lafayette Escadrille using 25 lb bombs in late 1916.
Re Orfordness trial - these appear to have involved 9 pilots not 5. Of these 4 pilots were testing Dh 4s using single relatively large bombs on special bomb racks. It would seem that the report was very detailed with graphs and performance charts on every type of aircraft and technique. However the body who seem to have taken most notice of it was the Japanese Navy who as allies received a copy.
Accounts of WW1 dive bombing may be obscured by the fact that at the time its often refered to as glide bombing not dive bombing (the engine was throttled back on entering the dive or even cut, the prop would then act as an air brake to slow the dive - WW2 dive bombers had purpose built air brakes - contrary to popular or Holywood representations dive bombers do not dive at speed - its the angle of attack thats important).