SUD:
Its irrelevant that the US can take Panama back. Its interesting that the US would divert a trans 2 inf and a plane to do so, since the whole point of the Jap attack is to pull US resources out of Europe. So a Jap attack on Panama is having the ideal result in this case.
If the UK builds an AC on turn 1, the US fighters are committed to fly to it and can do nothing in the Pacific theatre or else the German airforce will kill the AC with ease. At this point the Allied assault into Europe would stall for 1 maybe 2 turns. Against a good Axis player you have just lost the game. So by attacking Alaska, and you responding as you said you would will cost you the game.
The optimal response to the tired shuck-shuck strategy is for Japan to shuck-shuck into Alaska, and move its Airforce to Western Europe or Canada. The US will be tied up, and all the shuck shuck transports become wasted. The UK is at low teens income and effectively out of the game. Russia is caught in a 2 front war which it will eventually lose.
Japan can build an IC in Alaska, to build 2 tanks per turn and ship 4 to 6 inf per turn into Alaska while still pumping a tank and 4 inf into Asia. That’s enough for Asia because the only Allied player in Asia is Russia. The US will not be able to hard pressed to destroy the invading Jap forces. Any US ships lauched into the Pacific will be sunk by the Jap airforce in Western Canada. A force of 8+Jap infantry some tanks will take Western canada on J3. The US has to be careful, if it attacks Western Canada, it still needs to park enough forces in Western US to avoid a Japanese amphibious assault from Alaska to Western US. Most US players are not used to being invaded, and I think you would be surprised how hard the Japanese will hit you.
The Allied shuck-shuck strategy is flawed because it is so predicable, and easy to counter if the Axis works together. Germany sinks the AC on G2, and builds a fighter every other turn. Japan builds a fighter every turn and sends all its airforce to Western Canada/Europe. No allied ship in range goes unattacked. The Allied shuck-shuck stalls. Asia is now Axis dominated, and Germany has most of Africa. The allies are in dire straits because the UK has no IPCs, and America has no transports. Capital ships are expensive, and irrelevant. If the axis sinks all the transports, the allies are dead. Sorry I’m rambling.