1- Mopub is like a gateway for various ad providers, including admob ^^ There´s a guide on cocoonjs site of how to set it up.
2 - Yes, it provides the standar 320x50 and full screen banners for tablet and phones.
3 - I heard leadbolt and admob are the most profitable ;o
4 - Amazon, Play, Itunes App Store, Tizen. If you´re interested in desktop, gamejolt, kongregate and newgrounds are a must too ;D
Hope this helps, I had the same doubts a while ago ~_~