This is probably goofy, but in a situation like that I will say the word (possessive form) and if it, to me, sounds like there's an extra s sound on top of the ending of the base word, then I'll tack on a 's, otherwise not. (Option 2 is my default).
So you're saying that you'd say "Congress's", because the s is not an ending, but "Stevens'", because that s is already a possessive? That makes sense to me. I may do that sort of thing unconsciously.
That's largely how I do it, where, if it ends with an s that actually makes an "s" sound, then I'd use 's. But if it ends with an s that makes a "z" sound, then I'd use s'.
So "Congress's", because "Congress" ends with "s" sound.
"Stevens'", because "Stevens" ends with a "z" sound.
Poll
