Ed, you’re quite right to regard as grotesque the possessive of “Congress” with just an apostrophe and no final “s.” I looked at the article you mention by Starble in your update, but he says nothing about earlier justices. I can say from memory that the late Chief Justice Rehnquist habitually made this error. And it’s widespread in publishing: I had to cite Strunk and White myself in order to get an editor to use the “s” in my own work on one recent occasion.
So when did this mistake begin? You just used up an hour of my afternoon, Ed, but I can at least say that my hero Chief Justice John Marshall never used the possessive of “Congress” without an “s.” Oddly, however, this tells me nothing, for Marshall never used the possessive with an “s” either. He simply never, in 34 years on the bench, used the possessive of “Congress” at all in any of his opinions. He would, for instance, refer to a “power of Congress,” but never ever to “Congress’s power.” But I got the impression from my searching that this wasn’t just Marshall’s habit–that it was common in our early history not to form a possessive of the proper name of our republic’s legislature. I have no idea why.