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Usually if they don't keep clicking on your ads intentionally, there won't be any issue and no need to list each of them as test device. Further, you can connect to Google AdMob support if you have some exceptional case.
First of all, you should have Material+ version, after that, follow below tutorial:
You cannot do this to someone else's site, please do create for your own sites only.
No, you don't have to go back to homepage, you can double press quickly on any page to exit the app.
Yes, still double press the back key to exit from the app.
You need to debug your app to check console error and get to know the exact issue: