All longitudes whether absolute or relative are based on the tropical zodiac. In this system of reference zero degrees refers to zero degrees tropical Aries, which in turn corresponds to the sun’s position at the vernal equinox of the year in question.
Cerridwen’s source code is designed to be easily extensible to other planets and points. The goal is to add more planets in the future, probably starting with Mercury.
You’re right! At the moment the new and full moons are computed anew everytime, which is hard on CPU power. This will change radically with the next version of the module which will have a separate lookup table generation stage for these and other events. This will also pave the way for certain new features like the lunation number.