Couldn't the other shoe be their discussion over milk and coffee cake?
Where she's feeling resentful that the box was like a curse, or an evil power working through her, but as you said, RF is generous in his reassurance: "It hasn't stopped everything...It has stopped a lot...Now I'll thank you to give me the button box. Your work is done--at least that part of your work is done." He goes on to explain that she'll be successful and have things to tell the world, so how do we know she would have been able to do so without the box? Maybe that's the shoe. Maybe none of it was inherently evil.
It's not a bad read, but it feels like it was about so little in the end.
I think the hat comes from the character of Mr. Steward (from the story this was obviously based on, 'Button, Button' by Richard Matheson) who is said to wear a hat. Actually it was filmed twice, both for the Twilight Zone '80s run, and later as a feature film, The Box with Cameron Diaz by Richard 'Donnie Darko' Kelly.
To Matheson's dismay they changed the ending for the Twilight Zone episode which he had adapted himself, so he used his standard pseudonym of Logan Swanson for that. Actually I kind of like both the original and the alternative Twilight Zone ending.
The main difference between the two stories, is that the Matheson story is all about 'would you push the button if you didn't know the person who got killed and you got a large sum of money for it'. Gwendy already gets rewarded by the chocolates and coins, so it's only curiosity that tempts her to use the buttons, which makes the idea weaker overall.
And as for the R.F. initials: there was nothing really evil about the Richard Farris character, so I'd say that's a coincidence.
I just realized that these two stories were clearly linked. I knew Matheson had written the story that inspired The Box, and that The Box was pretty similar in certain ways to Gwendy's, but I just never made the connection. I see what you mean about Matheson's story being a more direct kind of tension--with more direct consequences--in a sense, but I think Gwendy's story is meant to riff on different ideas. As you said, Richard Farris may or may not be meant to resemble Flagg, and I thought the story was as much about the magic Farris controlled as it was about the choices Gwendy made.