Wow! I left for a little while and there are a lot of replies! Thanks for all the chatter; this is one of my first posts.
Okay. So to answer some questions and/or misconceptions:
The portal is sort of just a way to describe a theoretical idea of a "spawn point" rather than a literal portal to America. From a Rosharan perspective it's more like: "In that general direction is where all the giant ships and flying fabrials come from." As funny as it would be for Szeth to hold Nightblood under an open portal and form what would effectively be a localized black hole, that's not quite what I had in mind. I sort of assumed it was South of Kharbranth, beyond the mouth of the Tarat Sea, but it could be anywhere.
I also assume the United States has had spies and surveillance occurring for some time, but not too long, so anything that is common knowledge they know. Anything that is secret or even undiscovered, they don't. Although, I do find it likely that the affects of aluminum would be discovered eventually, if not immediately.
Some arguments of my own:
Soul casting is primarily based on perception, correct? Well, if the equipment of the United States even had beads in Shadesmar in the first place, how would it all be perceived? By the Rosharians, as dangerous, hateful weaponry slaughtering their ranks; by the Americans, as trusty, reliable hardware they use daily. A loyal, reliable soul with a superiority complex that is highly polarized against you? That's some of the hardest stuff to soulcast. It's why soulcasters don't just go to the front lines and turn everything into grape jelly already, in-world. (I could be wrong, so feel free to correct me.)
Lashing is really Stormlight-intensive and probably a big problem for lashing huge amounts of metal at once. Dalinar's perpendicularity can't be open that long, or give that much power, and if they did it during Highstorm hours it's unlikely there'd even be any ships to find in the open ocean.
Windrunners and the Heavenly Ones fall to radar-tracking autocannons (Like my profile picture!) and most modern military aircraft (Which can detect enemy craft around the curvature of the earth), because it's hard to lash something you can't even see coming because it's moving so fast. I think that'd apply to most ordnance, bombs and bullets included. Ordinary rounds can down most other Radiants, simply because head and organ shots are most always kill shots.
And for some fun aerodynamic reasons, the 20mm ELC rounds most commonly used by the Phalanx CIWS (But also compatible with C-RAM) are actually aluminum tipped, so they can annihilate even live shardplate in micro-seconds.
To answer your question, my friend, It is simply because I'm a massive nerd, and when I see a detailed fantasy map, with cool magic to boot, I always find it fun! It more grew than appeared suddenly.