Can the expedition leader also be a broker?

Solution 1:

You can pick anyone you like as broker, including the expedition leader. In fact, many of the social skills important to the expedition leader are also important to the broker, so it makes sense to use one dwarf for both. See the wiki page for the expedition leader.

In my experience it is most prudent to invest only a single point in the necessary social skills (if you choose to allocate points at all). You will likely get a migrant with better social skills for expedition leader/mayor/broker in an early migrant wave, so it would be better to invest those points in food, drinks, or other supplies.