I believe the clustering of nebulae is deliberate, from what I remember systems close to one with a nebula are more likely to be in a nebula
I believe that you are correct. There is a chance (around 25-30% I think) that any jump point explored from a nebula will connect to a nebula system with a similiar density.
Also the mineral generation chance is played with to give more mineral rich systems in a nebula. The downside of a nebula is you will never find more than 1 star so there are no binary, trinary ect type systems.
Brian