.."Overall, I'd say that Christian nations and Hindu nations (India and Bali) appear to be the happiest."
I fully disagree.Look at Philippines,my homeland, boasted as the only Christian nation in Asia.It seems to me that the Philippine people are in great suffer from poverty, economic crisis, terrorism and conflicts among people. Are they happy? No,the same thing with other nations(Buddhist or non-Buddhist countries) are experiencing the same problems or i would say our problems as world citizen. All religions teaches good on Human life. But you may find Buddhism "religion that exist for the sake of people" rather than people exist for the sake of religion.
Perhaps, you might have judge the superficial appearances of rich and wealthy nations(mostly dominated by Christian people) and you seems reasonable to be that the people living there are happier than people living in a poor countries. Neither material things, fame, wealth, status nor beauty bring us to real happiness or they can assure us of a happy life. This because happiness based on these things is relative happiness. It is depenndent, circumstancial and temporary. Whoever strives to construct a happy life based on wealth, status , fame or beauty will eventuaally encounter dissatisfaction, loss, and unhappiness. Instead Buddhism expound a happiness that is abosolute and lasting happiness. Absolute happiness is a state of life in which we enjoy our existence under any circumstances. The state of absolute happiness is also called Buddhahood.
Also, If by poor you mean economically poor, then it is true that some Buddhist countries are poor.But it does not mean or justifiable that they are unhappy.And if by poor you mean a poor quality of life, then perhaps some Buddhist countries are quite rich. America, for example, is an economically rich and powerful country but the crime rate is one of the highest in the world, millions of old people are neglected by their children and die of loneliness in old people's homes, domestic violence and child abuse are major problems. One in three marriages end in divorce, pornography is easily available. Rich in terms of money but perhaps poor in terms of the quality of life. Now if you look at some traditional Buddhist countries you find a very different situation. Parents are honoured and respected by their children, the crime rates are relatively low, divorce and suicide are rare and traditional values like gentleness, generosity, hospitality to strangers, tolerance and respect for others are still strong. Economically backward, but perhaps a higher quality of life than a country like America. But even if we judge Buddhist countries in terms of economics alone, one of the wealthiest and most economically dynamic countries in the world today is Japan where 93% of the population call themselves Buddhist.