Hey!
probOrbits is the probability of any objects in orbit around the star at all. Or, another way, 1 - probOrbits is the probability they system will be completely empty.
minOrbits/maxOrbits is the minimum/maximum number of things in orbit around the star. This includes accretion disks, rings, asteroid belts, and planets. And probably a few other things I'm forgetting. Note that for example a nebula/asteroid field at a Lagrange point, or moons, do not count against this limit.
habZoneStart is where the habitable zone around this star is. This is what habOffsetMin/habOffsetMax in planet_gen_data.csv is relative to.
As far as making habitable planets more common, that could get a bit tricky, since the modifiers for planet probability by star are in columns in planet_gen_data.csv. I don't 100% remember if it's possible for mods to add new columns - thinking about it, it may not be - so then the only way to do that would be going through all PlanetGenDataSpec in code and adding to their "multipliers" map, which is keyed off the columns in planet_gen_data.csv, but can be added to.
