I don't understand what you're saying. On the one hand, you're saying that John has a problem or struggle with grammar and that the placement is crude or perhaps less formal, but on the other hand you're saying that it is not a mistake and still acceptable. So what is it? Good grammar or bad? And if good, does the difference in position contribute to a difference in (semantic, pragmatic) meaning?
Good grammar for whom? By way of an analogy, William F. Buckley, Jr. used the English language in ways that most mortal's are incapable of duplicating. His grammar was probably impeccable. My grammar is simple, not sophisticated. I use a different set of criteria when evaluating Buckley's grammar than when I evaluate mine. Yes, John's grammar may not be "technically" "correct," but nor do I expect him to write at the level of Buckley (pardon the crude analogy, not to mention the chronological "incongruency"). Why do we expect all biblical authors to have near-perfect grammar? I don't believe language works that way. It is much more complex than just a set of grammatical rules that are to be obeyed.
I am really asking a question myself here. Was there one, perfect, ideal, acceptable set of grammatical rules in first-century biblical Greek? Did the writer of Hebrews follow "those" rules better than John did? Are some usages of John's grammar a "mistake"? Do we interpret John's Greek differently than when we evaluate Tertius' Greek? I have a lot of questions here but I really don't know where I come down on them.
You asked, "So what is it? Good grammar or bad?" Do you mean correct grammar or wrong grammar? Help me understand how one is to understand/interpret John's placement of a conjunction compared to the author of Hebrews' usage of a postpositive? I don't know. I don't see any significance with John's placement of the conjunction, whether in the second or third position. I understand, I believe, what he is writing here. Does the "difference in position contribute to a difference in (semantic, pragmatic) meaning?" No, not with John. As we read all of John's works, I really would not see a difference here. Do you? What possible difference could he be making with this "incorrect" position of γαρ. Does he demonstrate such precision in his other writings as to warrant seeing some semantic/pragmatic difference? By my saying that John's grammar may be crude at times was intended to make a statement about all his writings seen together. John does not write at a level of grammar that would cause us to look too deeply at this third position of γαρ. If we read an Atticist, perhaps we would ask such a question. As you can see, I'm just rambling and trying to write out loud about what my own questions are. Am I way off? I am always open to correction in my thinking.