Thanks for the guess on the Americano's weight. It sounds overbuilt for general touring with moderate loads. I guess it depends on where and how you tour though. On rough roads with heavy loads it might be just the ticket. Personally I'd rather keep the load light enough to not require such a sturdy mount.
I would also guess the frame weight difference between the Americano and Northwestern to be less than a pound. I suspect they use tubes maybe .1 or .2 mm thicker on the Americano. Like 1.2 compared to 1.0 thickness. As you state the extra toughness may be nice if touring from Prudhoe Bay to Tierra del Fuego or in Asia. Lot of unpaved roads there and it may not matter how light your load. Of course for Asia Africa N-S America touring, the Americano has disadvantages that would rule it out for me no matter what the cost. 700C wheels instead of the easier to find 26" mountain bike tires. Harder to find replacement tires. 145mm rear spacing, tandem width, instead of the common 135mm width for mountain and touring bikes. Replacement wheel would be harder to find.