As far as I see it, there are two options when London falls to Germany.
1) Japan goes b@lls to the walls after Russia, possibly tossing a major complex in Korea so they can start blitzing along the northern route. This maneuver forces Russia to pull out of Europe and protect their flanks, but it gives up China to the Chinese and the DEI to the Indians and Australians. Of course, this also means that the United States can focus hard on Italy either through N. Africa (preferred route) or C. Africa (less idea, but not horrible route.) Eventually you should be feeding US reinforcements into Russia from the middle east.
2) Japan focuses on winning in the Pacific which probably entails taking out China, India and the DEI. In this scenario the United States has to follow a more traditional strategy, but it also means the Russians can beachhead SE Europe. In this case, the United States should probably bypass any assaults on S. Italy, and instead focus on dropping reinforcements into SE Europe to bolster the Russians. Russia, meanwhile, has a prime location to move into the Middle East if need be (through Turkey) or just chew up German real-estate and make booku bucks.
The biggest threat really is a potential German invasion of Arkhangelsk or Leningrad. Both could be countered with a build or two in Moscow (and keeping in mind Russia should have Romania, Greece and Bulgaria as permanent holds, trading Albania, Yugoslavia and Hungary for an income of 48 IPC. W/O Japan in NE Asia anyway.) I would also expect the American Air Corps and/or Submariners to take out those Transports pretty early in the game and end that threat to Russia.
I guess really my thoughts on Sea Lion are thus:
Germany spends 2 full rounds of income to take 13 IPC worth of income. (England - 6; Scotland - 2; NO - 5) and it costs them at least 16 ground units, at least 2 or 3 air units and round 1 and 2’s expenditures when the Americans come to sink it all later. With as hard as it is for Germany to take out Russia without doing Sea Lion, I just don’t see it being any easier after they do Sea Lion.