You are correct. A single-ply denim is all that is allowed in the USAPL, unless it is an IPF qualifier. Then denim is not allowed. At least this is my understanding.
As far as the Westside/Radical cut is concerned, I was under the impression that Inzer's single-ply denim was not made this way. Perhaps their policy has changed. In the past the arms of a single-ply denim came out of the sides rather then the front (Radical Cut) because they were afraid that there would be too great a stress placed on the chest area and the shirt would subsequently be susceptable to tears.
I have not tried a single ply shirt, so I cannot compare it to a EHPHD, single-ply poly. I have heard that the denim was superior. I know that I like the double-ply denim much better. Not only do I get more explosion from the shirt, it is also so much more comfortable. My armpits do not get torn up and what not.