I'll weigh in first.
Kudos for thinking out of the box.
Your American plan depends greatly on what Japan does first. You will need a plan B and C.
You're dreaming if you think there is any way you will be assaulting Japan on round 2 with America. However, I like the fact that you are thinking all out attack on Japan immediately with USA in 1942 scenario. Japan is vulnerable. There is a lot of money in the Pacific. I usually prefer to take a Southerly route through the Pacific. NW Canada is an excellent place for land units, fighters and bombers. The infantry can get to Eastern Canada in one move for potential transporting to Europe/Africa, and all units are adjacent to the same sea zone as California so can be transported and bombers pose heavy threat to all Japanese boats.
Germany strat I like better. UK is vulnerable early on. Russia can be hard to take down. Again, I would wait and see what Russia does round 1 before buying 3 transports. I always buy a carrier round one with Germany and load it with fighters. UK would be crazy to attack you with two fighters and a bomber against a loaded carrier, destroyer and cruiser, so you don't have to worry about that. That said, you are correct that 4 inf, 4 tanks, 5 fighters a bomber and possibly a cruiser bombardment would be tough to defend. If your opponent is not wise and builds a carrier and flies his planes onto it, you will probably take over the UK round 2. But only rookies would do that. Also, it is a risky move if you fail.
If I was Allies and you made that German move, I would have a destroyer on the East Coast of UK to stop bombardment (you could attack it with Italian fighter, however). I would buy several land units, including some tanks (probably 6-8 altogether), to answer your threat and also because I will be needing them anyway for future amphibious assaults. I would leave the 2 UK fighters there. I might land the USA fighter there, too, depending on how much UK actually bought. I would probably even land 4 USA land units there as well with the starter transports. I would probably land 1-2 more US bombers there too. At this point, you would need jet fighters tech to even think about assaulting UK. And keep in mind, I'm lucky at rolling AA fire
In conclusion, I think it's a good plan to quickly topple an opponent that doesn't have too much A&A under their belt. But for any seasoned player, I'm afraid it is a somewhat weak opening move.
Welcome to the forums, and I hope you post more!