It seems to me that if you don't know, how are your students supposed to know? Whether or not this was the "right" thing for them to do, it seems to me that the students have a reasonable expectation that they won't be penalized for conduct that's not forbidden by their syllabus/honor code or grossly dishonest. Clearly, neither of of these apply here.

Most code isn't trivial. If you are spending a significant fraction of your time writing trivial code, something has gone wrong. Automate it.

You may also want to check for any errata that's been published for your textbook. It's possible the textbook authors agree with your professor.

I know that my school has a policy that forbids any grade (even a grade entered in error) from being revised to be lower after it's finalized.

@user3710669 unless that game you linked is open-source (and it doesn't seem to be), it looks like your classmates must have written all the code themselves. I don't see anything wrong in modeling your game mechanics off of someone else's work, unless that's something your class explicitly forbids. Especially given that the game is intended for children, I can't imagine that "novelty" is more important that "good execution."

(and it now reads) "However, I was unable to find any literature which discussed a link between Leibniz, this technique, and the early history of graph theory, so I am unable to evaluate the merits of this claim." I can't imagine there is any deep philosophical truth to be uncovered there, but I think it's a noteworthy remark in a throwaway undergrad midterm paper.

My paper is a trivial proof that propositions in Leibniz' logic can be encoded in a certain kind of graph. My background reads something like "Here is the status of Leibniz' work among some modern authors. The proof that I am about to provide you probably bears no relation to anything that Leibniz thought, since it relies on math that he did not have. Interestingly, however, this passage of Couturat {quote...} points towards some primitive graph-theoretic notions in relation to logic, suggesting that Leibniz may have had some ideas in this direction."...

Purely as a point of interest -- I the idea which I believe myself to be "bumping up against" is so tenuously-supported that I'm not shocked that I can't find anything (at least in English, my impression is that most serious work on Leibniz is in German). My main source is Couturat's "Logique de Leibniz" -- Couturat describes a part of Leibniz's combinatorics as being inspired by earlier work of Ramon Llull, who calculated (9 choose 9) by counting the number of lines he could draw between pairs of 9 items (which is exactly the complete graph on 9 vertices)...