As for Jump Points, the way I see it is that the wormholes are some sort of by-product of stellar formation. Basically, any time a huge gravitational source such as a star comes into being, the relativistic bending of space around it creates a number of wormholes whose other ends attatch to the strongest nearby sources of gravitational attraction (i.e. stars that are either nearby or that are farther away but super-massive). The maximum length of the wormhole then is a function of the product of the masses anchoring it on both ends, which explains why the Enigma black hole could allow somebody to cross the sector in two jumps (one jump to get to the Enigma system from anywhere in the sector, and then one more jump to get to any OTHER system in the sector).
An interesting case exists in that of Firekka, where there are many jump lines that lead back into the system. One could surmise that Firekka was formed in a dense star cluster, and then drifted away from it. The result would be that many jump lines "snapped" from being pulled beyond the distance limit imposed by the masses of their anchor stars. These "free" jump lines then sought new attatchment points, and those which could not find new stars to link to ended up linking to each other instead.