1. The benefits of a hunter gather type of lifestyle could be the closer we are to our food creates a better quality of food we can usually get. If we have to chase our protein across miles of woods we expend calories for a comparatively low gain which could quickly take care of any obesity problems. A nomad lifestyle allows us to leave or fallow the food and fallow the growing season rather than being devastated by drought you can pick up and scurry on off. Monetary value also crashes because an economy weights heavily on the money and taxes we pay for food. While an agricultural society there is more stability in larger groups where as in larger groups HG societies can quickly be literally screwed if game is not plentiful, as a hunter I can assure you typically game isn't plentiful. Having agriculture also allows for a more stable economy and import and export.
2. Some of the disadvantages of HG lifestyles is the fact if there is a large population to sustain you can deplete the game in the area and if there is no game you will starve. In a HG society those who are not as agile or strong are not viewed as useful and any with any serious issues can hold the group back. A huge disadvantage of agriculture is the fact the food is so weather dependent. If weather is poor or there is a blight it could be a repeat of the Irish potato famine.
3. The healthiest diet depends on your living conditions, hunter gathers tend to be healthier but overall I believe the best would be a mix of the two.
4. I think early humans made the transition because its simply easier to supply a population with a steady almost predictable amount of food. For a population which is growing rapidly this a huge plus and give them stability in a unstable world.
1. Your population has a minimum need to survive what ever isn't used is a surplus which can be readily traded. Its a dangerous game to trade on supplies that could possibly be needed and you could end up starving yourself just because you canted to trade without having the proper resources at your disposal.
2. A social benefit of trade is the exposure to other cultures which allows different thoughts and views on life. Along with expose to other people you also gain new technology passes on similar to how it was on the silk road.
3. Trade gives a some people all the power and some people nothing due to the uneven distribution of wealth, and the wealthy can pick on the poor. Another negative could possibly be exposure to new pathogens that the native people do not have antibodies for could cause a epidemic.
4. Only some foods will grow in some areas so if some people have things others do not but those others want it they can arrange a trade, simple supply and demand.