That word order could not be called "valid" by any stretch of the word. The only valid word order is as you said -- 「メンバーの都合がつかず」.
Despite the random word order, though, the phrase would be understood by virtually all Japanese-speakers because it is short and it contains all of the necessary words for it to make sense.
How the "mistake" happened, I have no idea. 「都合がメンバーのつかず」 makes no sense as-is and it is not something a native speaker would say knowingly. The only possible words that could grammatically be placed between 「都合が」 and 「つかず」 would be those that function adverbially to modify 「つかず」, such as 「うまく」 and 「なかなか」. 「メンバーの」 is not such a phrase.
I would imagine that a mistake occured in the process of editing as you suggested. They might have tried to insert the 「メンバーの」 part later and ended up inserting it in the wrong place.