I think that your emphasis on current common usage is treating a moving target as if it were fixed.
Groups have their dialects. If I wanted to talk to a Southerner I learned what he meant by "jitney", "carry", "poke", "buggy" and the rest. And after a while I found myself using the words as they did.
If I want to talk to a sailor, while he should know that what he calls a "line" is generally called a "rope", and "port" is not just an opening or a fortified wine but it is also a direction.
And here's an instance: TO me "mad dog" means a rabid dog. I took the name because I was exposed to rabies. But if you mean a psychotic dog, I can work with that. Just letme get a note from my pshrink.
I think there needs to be give and take. Part of that will be openness on the part of all parties to the idea that the other party was using "shop talk" wiuthoutmeaning to or knowing that he was doing so. I have NEVER (that I can recall) taken 'co-' tomean 'equal'. Even among co-authors usually one takes the lead. Put that on your ISP and save it.
And a co-pilot is still subordinate, is s/he not? He is not equal in rank or responsibility to the pilot, unless something happens to the pilot. If it comes to orders, the pilot gives them and the co-pilot takes them, is that right? I think it is.
How do some of these rubes explain the word "coequal"?
Regional differences in word usage are not the same as jargon, the specialized language of a particular group.
” Even among co-authors usually one takes the lead”
Indeed so, and likewise among co-pilot and pilot, the pilot is boss but the author's or pilot's mother doesn't become co anything by virtue of carrying someone or being supportive of their work. The mother deserves much, certainly the Wright Bros. mama or Twain's does for whatever part they had but they're not co’s of either.
If the current and common usage of co changes I will go with that, after all I gave up shambles for abattoir.