1. Yes, just that's mostly if you go directly, without mediation. This way I will have to call a real ad, so there is a chance that test ad by Vungle or Unity could be received. Otherwise I will get a real ad from Google Admob, which is bad for now.
2. One account or more, it doesn't matter here. I have dealt with everything like scoring and all other things even if one user has more than one account. One thing I can thought of is the one I said - Username, the attribute that google ads follow (phone, ip or whatever) and time difference. Btw, what if I read the device? That's not okay I think. User can login on another device and click the video again if google follows the device. IP also, what if two players on two difference devices share the same ip? They both should be able to click the video on each account/phone. Maybe google follows several attributes, not only one.