I'm in total agreement with JFM and others who find this kind of a pointless exercise, involving far too much speculation. On any given day, if someone achieved surprise and luck, they could shoot down an experienced ace (think Avery vs. Menckhoff). As the Germans say, "One must have luck."
Udet may never have fought McCudden, but he did fight a lengthy combat with Capt.
Tom Falcon Hazell, DSO, DFC, leader of 'A' Flight of No. 24 Squadron (with an eventual 43 victories) in his SE5a, on a blazing hot 22 August 1918. Udet was flying his BMW-engined Fokker D.VII F, and Hazell was in SE5a B8422 (though McCudden didn't survive long enough to have fought with Udet in a D.VII, perhaps this is a good analogy). I believe Russ Gannon and/or Peter Kilduff was the first to figure this confrontation out. Udet wrote eloquently about this hard-fought struggle in
Mein Fliegerleben, and so have Peter Kilduff, Jon Guttman, Mike O'Connor and I in our various books. Udet arrived just a bit too late to prevent Hazell from buring a German balloon, then chased the SE5a home in a long and harrowing pursuit at tree-top level. Udet thought he had shot the SE down (and was credited), but Hazell managed to make it back to Bertangles. To quote the 24 Squadron report, "Capt. Hazell was then seen home by the enemy aircraft, who shot his tank, longerons and propeller to pieces." The SE was soon struck off strength as unrepairable. Udet, on the other hand, was then attacked by three SE's of Hazell's Flight, and one of them chased Udet back. In the heat of the August day, Udet saw that his phosphorus ammnition was starting to ignite. He quickly squeezed the triggers to get rid of the unstable ammo, and was shocked to see the SE5 pulling away...
So both pilots survived this harrowing combat. That's why I like it.