It pays clicks no matter where in the world it comes from but US clicks are reportedly more valuable than international clicks.
Adsense is Geo-Targeted. It all depends on the advertisers. I have international traffic and I do make money using Adsense.
Yes, of course. Advertisers on Adwords on the other hand can determine which region their ads can be shown so there is a chance that your non-US visitors may not see the more US-centric ads which may pay more.