So, you found out after the fact that the 
  global adversary injected themselves in 
  the middle of your conversation with a 
  source. What do you do now?

If you could reliably detect a CA issuing MITM certs to the global adversary, and if some unstoppable mechanism would respond to such detections by promptly dropping the CA's cert from clients' trust roots, and if being dropped from the trust roots put the CA out of business, then it would be extremely difficult to induce a CA to issue MITM certs.