I am still interested in help on this one.
1. Community specific attributes such as parent/child relationships is not available in 3.0. However, these attributes have been brought into analytics in 4.0. There is a new dimension table called jivedw_community which contains the lfg and rgt columns from jivecommunity.
2. You are correct that it is the lft and rgt columns that are used for retrieving the descendants on a community. In order to select all descendants for a particular community you would perform a query like this:
select communityid from jivecommunity where lft < x and rgt > y
where x = the parent community's lft value and rgt = the parent communty's rgt value.
If you want to include the starting parent community itself then you can change the comparison operators to <= and >= respectively.