By fake ads I mean simulated ads, non real, just gifs or jpgs inserted on the demo sites. This serves the purpose to show the material and won't compromise anything. Is very important to include the text "demo" somewhere on the ads to avoid problems or being accused of falsification.
Flight club has been mentioned several times as a joke but also as a very true comparison to Adsense. We can't control who visits our sites but is advisable not to tell others this or that is my website with Adsense on it. Here some webmasters shared how they had problems after telling others about their web, as their friends began clicking the ads (the same could be done by your competitors), I don't know for sure how that can affect the sites today but is better not to find out.
I get your point but honestly I don't see any reason to put real adsense ads on demo websites other that getting some income, but anyway it means a risk in many ways from my point of view as a demo site won't have real or accurate information, and that could be the source of your problem. Instead, why not creating real websites instead of demo? See it from their perspective, MFA that copy and MFA that invent their info.
Hope you can get your sites working with AS again. Good luck.