The sole principle of the doctrine is the law of comparative advantage. The only fundamental condition that needs to be fulfilled to maximise the gain from free trade is that markets be free. Comparative advantage is a very difficult concept for most people to grasp, including Davidson I fear. An example would be that even if a country could produce a number of goods more 'efficiently' then its trading partners it would still pay that country to import those goods if its opportunity costs of production, the value of the alternative production it has to sacrifice to produce those goods, exceeds their value. The effect is to rearrange production in away that raises total output.
Full employment is irrelevant.
However, creating widespread unemployment by overpricing labour, as Australia has done, will certainly reduce the benefits of free trade while giving protectionists like Davidson an excuse to justify tariffs. This means, according to Davidson, that the consumer gains from the lower prices of imported goods would have to be offset by increased taxes to pay unemployment benefits and cover the loss of revenue from fewer tariffs and a smaller tax base. I would not accept this nonsense from a third rate first year undergraduate. Australian consumers pay the tariffs which means that their abolition would not raise aggregate taxes but only change their incidence and probably bring new ones into existence. The solution to the so-called revenue 'problem' is to impose a uniform tax on all of the products in question.