"If variation developed after Dacia and Dalmatia had been effectively cut off from linguistic innovations from the west, these areas would never have picked it up."
"If A happened after B had been removed, C would never have noticed." ->"If A happened, C would never have noticed."
If you want an opinion about the structure, I would write "If A had happened / If variation had developed ..."
"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).
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'? 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.