If both legit and ad urls all look the same, and can only be distinguished by decrypting with a key, then adblockers may be unable to differentiate between the two.
Your can differentiate with a list of object SHA hashes you blacklist based on ad blocker user feedback. You'll still need to fetch the object, but you can dump it before rendering.
Excellent point - you could monitor the ABP database and if the hash appears, modify the content (shifting the value slightly on a single pixel) so the thieves need to block the new one.
I assume by thieves you mean ad networks, because I never agreed to retrieve their content, let alone view it.
Its an arms race, as always. And just as the media industry couldn't beat piracy, ad networks aren't going to beat blockers, even if it means content producers get their content stripped and distributed via other channels.