mjhjmtu

Note: I already reviewed many topics here regarding the use of わけ, but I still struggle to properly translate the sentence below. It's from the introduction to the book 日本語文法ハンドブック (a book for teachers of beginning Japanese grammar).

言うまでもないことですが、現実の日本語に初級と中・上級との区別が存在しているわけではなく、初級の文法というのは、いわば指導上の便宜的な設定です。

Translating literally produces something like
"There's no need to explain it, but for real [actual-use] Japanese, it's not the case that distinctions between early and middle/advanced Japanese grammar exist, in other words early Japanese grammar is setting an expedient foundation [for teaching someone to learn]."

Which sounds like utter gibberish to me. The stumbling block is the phrase "初級と中・上級との区別が存在しているわけではなく". I would think "存在していないわけではなく" makes more sense ("it's not the case that distinctions between different levels of grammar don't [already] exist"), but that's not what's used here.

What is a correct translation of the sentence above, especially in regards to the ~わけ phrase?

Read it again -- the author is saying that there are no distinctions between "beginner / intermediate / advanced" in the real world of actually-spoken Japanese. Same for any language. Such distinctions are convenient fictions created for the purpose of taking a vastly complicated system -- a language -- and divvying it up into manageable pieces for purposes of learning and instruction.