If I had to guess I would say cancelled is when you X it or close the app instead of watching it to the end, and completed is when you watch the full video and reward the user.
Second question is interesting. Test it out. If you create an ad, can you load it on other layouts? After the first time is it loading instantly? You can only create one of each ad type at a time. My guess would be when an ad is loaded and ready it is ready for the entire runtime.