I've never heard that YPN pays more than Adsense. But even if you do switch, it is really gonna depend on the keywords that are used for the ads.
There is no right answer - it depends on your site. The only way you are going to know is to apply to YPN, and if accepted, try it and see the results. The CTR is usually lower with YPN so even if the click price is higher, the net might not be. You can only show YPN ads to US visitors - something else that factors in.
There is nothing like that the ads are shown only for US Visitors. Only US Visitors can apply for YPN since it was still in beta.
You should read the TOS. Abuse of Services. You agree not to: # display all or part of the Ad Unit to any user located outside the US; https://publisher.yahoo.com/legal/tnc.php In practice, YPN probably isn't going to terminate you for a small percentage of non-US traffic - but it gives them that option, and there are MANY publisher who have already been terminated for non-US traffic. Use Geotargeting to remove the termination risk.
well depends on what niche and keyword you are talking aobut. i think that adsense is better, even if they pay a little less - they accept clicks from many countries.