ALso, about the food analogy, we're assuming that there is only one product in the economy.
In reality, there are many different products in an economy. Economy B really won't have a lower GDP because its citizens are going to spend the extra money that they saved on their grocery bill on hookers and cellphones and stuff. And since hookers and cellphone providers are a higher-markup industry, that money is giong to get recycled to create even more GDP.
Assuming they spend everything, yes. In theory, a percentage of people will save some of the money that they didn't spend - and by doing so, will decrease the volume of money traded for goods and services, and thus also reduce the GDP even if it is by a very small margin...