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.
Not already a possessive but sounding it out (of course this presupposes that I pronounce words 100% correctly every time, which I think I do, but you can question that)
Poll
