What are all the steps to be performed, from the beginning;
You need to register an account with an ad network you like to work with. Remember, not only Admob but also many ad networks overthere, such as StartApp, MobiCore, Applovin, Leadbolt, Inmobi...
How to recognize if the user has skipped the video (how to intercept keys) to know if you should reward the user or not.
You use ad network's SDK and they provide method to show ads, events to know when the ads end, to know when users watch it completely or not.
Again for all I know, Rewarded Videos (or shoud I write "Rewarded Interstitials"? Are they the same thing?)
Interstitial ads are full page ads, it can be text/image/video/interactive ads, you earn money when users click on it or when you show a large number of it. Rewarded ads are interstitial ads (again, it can be text/image/video/interactive ads), when the users finish "a mission" (watch video / download app / finish a poll...) you give them award (coin / life / blood) and you earn money from ad network.
Scrabble's videos were 3 (I played a few minutes): Lavazza, Vogue and Air France; how to get these important brands? EA, which developed Scrabble, may have contacted these companies directly? Or it has chosen a particular Ad network (and how)?
They can use their "internal framework" to choose which ad will be shown. But I belive more that they used mediation ad network. Mediation ad network sends ad requests to multiple ad networks and find the highest paid network to fill ad slots. There're many mediation networks such as MoPub, AdtoApp, Appodeal and AdMob too.
If you want to use mediation ad network, I recommend to try Appodeal (they provide B4A wrapper) or MoPub with Johnmcenroy's wrapper
here.
why (how) a user was able to gain money without having a "Google Merchant Account" using banners only? Paid directly the Ad network? And if so, holds Google 30% anyway (and how)
You don't need to have a Google Merchant Account to be paid by ad networks. They used to pay through PayPal, check, bank account or Western Union (AdMob). And depends on each ad network, you need to earn a number of money before they can pay you ($50 for StartApp, $100 for AdMob...)
PS.1: My suggestion, instead of just sitting here and ask many questions, why don't you register an account and try by yourself? Of course, it's free and it's really fun when you see the money rise (even just a few cents).
PS.2: Please pay me $100 for consultancy fee.
PS.3: I'm just kidding, good luck!