If you wanted to re-tweak this, you could just give them the $64 IPCs and let the design the force that is up there. However, there would be an optimal way to do this, which would be quickly discovered, and it wouldn’t be so much flexible as obvious (eg, buy 5-6 fighters, send em home…)
The pieces here aren’t a threat because there aren’t any mobile, threat projection units with them. So, Japan isn’t too scared when you leave them nearby, and they have a diplomacy rule to hide behind (loss of Mongolian help).
Really, you need to block Japan and slow him down as much as you can, 1 guy 1 territory at a time. Having those guys head back for Moscow has an additional plus; if most of them survive you have a force waiting in North Central Russia that can counterattack Japan as they try to rip Russias backdoor apart and take all your $$$.
As you are stating, the fact that the submarine war (and the blitz) appeared to be putting the UK close to the ropes was a MAJOR factor in ending US neutrality. The US began “neutrality patrols” guarding ships carrying arms as far as they could justify, and right before the war, even attacking german submarines carrying out “legitimate attacks” on UK war shipping.
German repeatedly protested this “interested neutrality” in both world war 1 and 2. The US hid behind this to ship arms to multiple belligerent parties, of its choosing and in its own interest, which is about as not-neutral as it gets. That’s some “Special Relationship” as the british call it.
As I’ve pointed out before, I believe 1-2 US destroyers were sunk and had begun depth charging german contacts before December 7, 1941, so the German submarine captains already understood that they were standing on the line between war and peace.
“In the “Greer Incident” on 4 September 1941 the destroyer USS Greer (DD-145) was fired upon with torpedoes by German submarine U-652.”
“Either the casualties inflicted on USS Kearny by German submarine U-568 on 17 October 1941 (11 KIA) or the sinking of the USS Reuben James by U-552 on 31 October 1941, (115 KIA) might be considered the first American naval losses of World War II. The United States was neither officially involved in the war at the time nor did the incidents cause them to declare war.”