I changed the Chromium browser (as a masters project) to intercept suspicious extension actions like inserting elements etc and to alert users of what the extension is attempting to do. Using this proof-of-concept browser would have helped you debug your ad injection problem!
https://cypher.codes/writing/intercepting-suspicious-chrome-...
- Note: my project specifically tries to protect users from Facebook hijacking and ad injection attacks - the two most common attacks on the CWS!