I wonder why the Shivan Nebula is called "Lupus Nebula" in that nodemap. Thought it wasn't on GTVA star charts? They always called it "the nebula" in the main game.
As for Ross 128 and Ikeya, I think that the only reasonable explanation is that there's an unstable corridor there. Think, if not, then the Lucifer would have had to pass through Delta Serpentis, one jump away from Earth. If it had come through that way, it would have explored the DS-Sol node and wiped Earth out. But, instead, it next appeared at Ribos, and blasted the hell outta Freespace's Pearl Harbor analog (Tombaugh Station). Also, why would the Shivans have gone through that one node into Ikeya, when it's a border dead end system? They must have come in from another direction.
As for "short walk to Earth", if you study the nodemap of FS1's final missions, it shows a jump corridor between Sirius and Delta Serpentis. Why they decided to get rid of it later is beyond me, but in that context it makes more sense, as Sirius is right alongside Deneb, which at the time was Shivan-occupied.