The Prudie column follows a rather standard formula that was probably devised back when magazines were invented. The basic question is this "How can I get someone else to do what I want?" There are themes that seem to repeat in these columns but they are all variations on that base.
Your two sources for "letters" are equally plausible (or implausible) depending on your general outlook on life. If you think about it for more than a second, it doesn't matter whether someone wrote in to "Prudie" or an other columnist to try and get "authoritative" support for what she wants, or whether Prudie or her staff have this list of snarky comments and they need "letters" from really dumb people that fit the snarky comments.
Either way, those of us who read this crap get dumber with every minute we spend here. Heck, I used to have an IQ well into three figures.