Bean, it seems your questions are, 1) US vs. UK taking Norway and building an IC there, and 2) Norway IC vs. just shucking with boats. On #1, it really depends on Axis strategy and the dice, to some degree. If Germany falters early in Africa, British income could potentially reach the mid-30s by turn 5 or so, in which case, Britain will have a little extra income to support a Norway IC. In some games, though, America ends up taking Norway through a weird set of circumstances, and when that happens, I am more than happy to build an American IC there. And why bother shucking over an American AA? Why take up the transport space? Just move over Britain’s and build another to replace it, if needed. If the German bomber is dead, though, don’t bother. So I guess in most games, I prefer UK to take Norway for income reasons. I don’t really go out of my way to let America take it unless, as you say, Britain is tied up elsewhere.
As for buying an IC versus just shucking, I look at it this way. To get 3 units from America to Norway, America must invest in 32 IPCs worth of boats (2 to get them to UK and 2 to get them to Norway/Karelia). Granted, that actually gets 4 units over, but that’s the investment that’s required. Compare that to 15 IPCs for the factory, and the fact you’re building on the front, and it’s an easy decision.