If you want an opinion about the structure, I would write "If A had happened / If variation had developed ..." [Comment: I also would do this]
"If variation had developed, ... these areas would never have picked it up."
The past perfect is needed here, in my opinion, and the 'after' clause has no effect on that. (As I explained a few weeks ago, I think). [I also agree]
Sorry if I've missed the point. Could you let us know whether this is your question - about whether it should be 'developed' or 'had developed'? [No, I am sure it would be 'had developed'] If it is, then the answers are going to be the same no matter how many sentences you find on the web. Isn't this the same structure you were asking about at the beginning - the one that was answered in posts #2 and #3?
PS: You wrote: "I believe formal contexts are void of such mistakes." This is a seriously erroneous belief, and the probable cause of why you haven't received a satisfactory response yet. People make mistakes. If you persist in believing that they don't, then no reply here will be satisfactory, and it's all a waste of time. [It is me who had been saying "To err is human" so I am not saying there is anything totally complete but I only wanted to say that these books are written carefully and proofread so the chance of there being such an unnatural mistake is very small.]