You are assuming the Germans are able to do ITTL what they did in OTL, considering the US has just ended a successful war in Asia, and has an army of veteran troops, that assumption is dubious. There is no reason to believe that the Japanese will fight with any less ferocity and intelligence in a "Panay War" than they did in OTL, the US Army will have learned many lessons and be much sounder in doctrine and tactics than either the French or British because of this.
I dunno. The shape of the Pacific War looks a lot more Hector Bywater than Plan Orange, if the US goes Ape over the Panay Incident. One thing is for certain, There is no Kido Butai, and hence no Pearl Harbor. Three years makes a huge difference in aircraft carrier warfare in the RTL. It could be that battles more reminiscent of the Mediterranean than what occurred in the Pacific result. In that case, the USN is in for a rude surprise as it learns its night fighting torpedo and gunnery tactics are not up to Japanese specs. It might take a year of Savo Islands and Kolombangaras (Second Kula Gulf for some of our Asian colleagues) for the USN to muster up. The shooting war resembles a one axis drive through the Gilberts, Marshalls, and Carolines to the Marianas. Would there be fighting in the Philippines? Yes. But since the Japanese have not built the amphibious shipping they have in 1941, the Lingayan Gulf fiasco may not be as bad. Could be a protracted fight on the central Luzon plain. MacArthur with a competent staff officer (Eisenhower) might actually not bungle the defense.
In any event, the US may indeed learn a few things.
On land: squad and platoon tactics. Cleaning out Japanese bunkers and revetments tends to make infantry intensive small unit tactics a primary focus. How this translates to Europe may mean a better US army NCO corps. That is a good thing. On the negative side, there won't be too many actions above battalion or regiment size to season the US army officer corps. The Americans should be expert in amphibious warfare and well equipped for it. They may actually have a good tank by 1939. (Bunker clearing almost demands something armored with a large caliber dual purpose gun. Might not be a Sherman though. Might be an American assault vehicle, like a Priest, but more armored and akin to a Stug.
At sea, victory will more than ever hinge on submarines as the carriers are not quite there yet. Could the USN go through its torpedo crisis and solve it earlier? With Harold R. Stark? I'm no fan of Stark. I don't think he would do better than King did. I think he could be worse. I mean Stark was Bu-Ord from 1934-37 and observe how he fared there. Most of the debacles for which Admiral William Furlong is unfairly blamed are Stark's screw-ups. Furlong was trying to fix these mistakes (those he knew about). He was succeeded by another disaster, the Atomic Playboy, Blandy. It might take hot-tempered O'Richardson to fix things, but O'Richardson and Roosevelt were like fire and a fuse train to dynamite.
In the air, the issue is can the Americans learn how to translate Pacific CAS to European TacAIR interdiction? Their air forces, naval and army, will be in this ITTL most likely medium bombers and single engine fighters. Being at war, a lot of shilly-shally will be brushed aside and planes will be rushed forward into service that might have never seen the light of day. F5F anybody?
Secondly, there is no guarantee, at all, that Germany will act as they did in OTL. With the presence of a US Army on the continent, Hitler may balk at moving through the Ardennes, and go for the original plan.
A US Army slid into say between the British and the French? (See map.)
I know it is near ASB, but it is about as likely as the OP suggestion about the Panay Incident.
Factor in the effects of having large numbers of working, well developed aircraft, communications equipment, and logistics (which will be essential in a Pacific War) available to the Allies, and German's chances of victory are considerably reduced. A good radio net (and the US will have that) alone will prevent the lack of information and paralysis that plagued the French during the war.
One cannot assume the French will be much different, but the UK/US team (about 600,000-900,000 in 3 field armies) will be. If nothing else, the Anglo-Americans will be trading notes in this ATL as they did in the RTL. One thing will be immediately obvious. Uncle will have artillery and "tanks". Lots of both. And airpower.
US participation in WWII at this early date opens many variables. Do the Belgians continue with their head in the sand efforts at neutrality? No US weapons then. Will German invade through Belgium? Holland? Or, more likely, continue to propagate the lie that Poland started it all, and hope to hold the Allies at the German border? (Which is what I think they will do)
I'm thinking that the British with an experienced US (amphibious warfare, littoral warfare, possibly carrier warfare, and most definitely small unit actions that are reminiscent of the kind of fighting the British experienced in and around Narvik on land.) at their back, make a much harder fight for Norway.
The Battle of the North Atlantic changes too. The British will insist on convoy in this war as the Americans insisted on it in the last war. Will the Americans listen? Depends on the Pacific War. I think their own USN submarine campaign against Japan will convince them.
Italy will take one look at the lineup and sit this one out.
Russia will also sit it out. Marxist theory *(Stalin believes in it.) is that the capitalist states will destroy each other. "Let's you and him fight and I'll knife the loser in the back." could be Stalin's game. It was with Poland. And essentially, if Germany had not attacked Russia, it would have been Stalin's preferred ploy to wait until Germany and the Western allies had bludgeoned each other senseless, to come in and swallow eastern Europe as far west as he could march before he was stopped. See further...
What the Soviets do is up in the air. It may well end up with an unholy alliance of the SU and Nazi Germany against the Free World, (and the Soviets will be paranoid regarding their Far Eastern Provinces, with a US friendly China sitting there) and the Allies fighting the two of them. If so, it will be the Napoleonic Wars writ large, a lengthy contest of wills. Eastern Europe stands to loose terribly in that conflict. They may try for neutrality, I doubt it will work.
If the Americans follow the script of WP-58, Japan is a goner by 40. It will be submarines and not carriers, and it will be a negotiated surrender, but the Americans will actually have an easier war as the Japanese will not have gotten into S.E. Asia and Indonesia, their air forces will not be as experienced or as powerful and their ace in the hole, the Kido Butai, will not be the sword and shield behind which they can conduct their stab at the southern resources area. People do not realize how important the Japanese buildup from 1937 to 1941 was in enabling them to run riot in the Pacific in early *42.
I estimate that a defeated Japan, an insurgent Nationalist China and a war experienced US in the Pacific; nudged right up against the Soviet maritime provinces (Aleutians) makes Stalin a very polite boy. I do agree it will be a very hard row for the Eastern Europeans. and probably the Germans, too.