First of all, you are in charge of your app and you business, so eventually the power is in your hands and any decision you make will be reasonable.
The 10%-20% fraud ads you saw came form the same account. As we are only into apps, unlike some ad networks, this is a very rare incident. So rare, in fact, that this is the first time I've seen this kind of fraud. To this day, there is not a single developer who got his app banned from the Appstore of Google Play because of Appnext. Not even one. You can check it in developers forum or anywhere else.
As for what you suggested -
A camaign cannot go live if the URL doesn't lead to the Appstore or Google Play. It's not possible. In this specific case, something else was done (I can elaborate in private), which we are wroking on to make sure cannot happen again. You definitely DON'T need to set up crawlers of your own.
If I were in your place, I would test us for a week (not even live, just on the request basis), and see for yourself that these kind of events do not happen often, or hardly ever happen. There is no other way for you to trust us.
Any more questions about this issue - feel free to contact me in private.
Director of Partnerships & Community, Appnext
| m: +972-52-3320008 | e: firstname.lastname@example.org | Skype: yoni_raveh