Yesterday I may have inadvertently e-filed another hypothetical tax return. If so, how do stop the hypothetical return e-filing?

Impossible.  You could not have e-filed a return yesterday on November 6, 2019.  E-filing has been permanently closed for 2018 returns since end of day on October 20, 2019.  So breathe easy.  You did not file anything.

I can tell you with 100% confidence   that you could NOT have efiled ANY tax return yesterday since the efile system closed 10/20/19.