A binary operation on a set is closed on the set by definition. If an operation isn’t closed, then it isn’t considered a binary operation on that set. Of course, it doesn’t hurt to state the closure property explicitly.
I have talked a bit more about it in a totally unrelated blog post here: https://susam.net/product-of-additive-inverses.html#closure-...
2) the reduction step up multiplication of nth order polynomials (to keep them nth order) is missing (or at least I missed it after a couple of readings.)
Apart from those quibbles, this was really good overall though. I enjoyed it.