I agree the notion seems impractical from a logistical perspective. For one thing, Swimtern plays a critical role in getting his uncle to go along with Plan Swappity-Do. It seems highly improbable that Sorkin was involved in developing the character's backstory over several episodes just for that moment.
On the other hand, I would definitely say The Supremes doesn't seem like a typical Debra Cahn product. She wrote two of my favorite episodes, Abu El Banat and Institutional Memory. They are both focused on relationships and the effect political careers have on people. The last few seconds of IM, when CJ realizes she doesn't want to work at the White House anymore, is one of my favorite moments in the whole series.
The Supremes is the opposite of that. It is packed with constitutional issues and the politics that go into getting new justices onto the Court. Mulready's reference to Harlan's Dissent is another signature moment of the series. I watched the episode again a couple days ago and continue to be impressed by it.
And then there's the casting. They got a lot of high-powered players for this one episode. Glenn Close(!), Bill Fichtner, Robert Picardo, Mitchell Ryan, and Milo O'Shea. Maybe they did plan way in advance to create this one swansong for Aaron Sorkin.
In the end it's impossible to know the truth. In the commentary Cahn mentioned having many conversations with the support staff about the political process. One also has to wonder if Sorkin could have come up with the idea of a compromise involving cookies. I don't really care one way or the other and I don't need to know the answer; for me it was more valuable to think about the question.