I think it would help for you to also hear from an Ad Network on this idea of yours.
What you’re proposing is not only going to get you in trouble with Google Play but also with the Ad Network that you use.
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Google-Play: As the policy points out, you cannot force a user to click on an ad to get some sort of a reward. Your argument about incentivized clicks is not very solid because the user has other options too. The user can ‘purchase’ the locked feature or in many games, wait a few hours to get the same reward without purchasing or downloading anything. So unless you’re giving out such an option, you’ll be in violation of Google Play’s Policy.
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Ad Networks: If your idea works out what Ad Networks will see is a big rise in CTR (click through rate) and every Ad Network flags high CTRs. That is NOT to say that you’ll get banned the moment you get 10% CTR with an ad. That won’t happen even with Admob provided your clicks are converting. Many Ad Networks and all advertisers track conversions (users completing actions i.e downloading an app or filling a form through the ad) and whenever there is very high CTR and very low to no conversions, Advertisers stop spending or block your site and the Ad Network warns you/bans you.
All in all, please do not do such a thing as at best it will work just until one of the three parties here flag you and that is going to take less than a day.
Hope that helps.
Thanks