...but what is the WLL of that tarp system? If it is considered "securement", it should have SOME kind of rating, shouldn't it? Otherwise, it isn't technically "securement", now is it?
Almost three decades now running mainly in the midwest have never been questioned by the DOT for. having straps outside the rub rail when I had loads similar to yours.
Even when the regs called for straps inside the rub rails, it included an exception which allowed the straps outside if the load was wider than the trailer or any other situation where it just wasn't practical to go inside the rails with the straps. These days, you're free to be lazy and toss the straps outside of the rub rail on every load.
Now this is off topic a little, are coils the only time chains are required? I had a 98" wide stack of steel plates with narrower ones on top, I did x chains across front and back then strapped the rest because the overhang on my 96" trailer made it hard to put chains across and make them feel secure without tearing up the edges.. the overhang didn't work will with those steel edge protectors.