Well, I can answer both of those, Mephiles.
Sleheyron was scrapped EARLY in production; the dialogue files weren't even the same format. It was scrapped because it was the least vital element and the team thought they were over-extending their workload. David Gaider designed Sleheyron(what little he got to do before it was scrapped), and he slid in Yuthura's lines about growing up as a slave on Sleheyron afterwards, to get his two cents in...
M4-78EP is only about 10-15% original story concept, and the rest is built off of that and the team's interpretations of side effects, consequences, and ways to flesh out the areas. The original concept was that Vash went there to investigate the missing colonists that had colonized the planet. The colonists disappeared and Vash, searching for emptiness in the Force, went to M4-78 to investigate it. It being a planet full of droids(no Force signature of "life signs"), she'd perceive it as an emptiness, a hole. Given the Exile's "disease", it makes sense why Vash would want to check it out.