HIPS is not water soluble, it requires a solvent, I think acetone works. Also, I do believe slic3r has multiple processes (I know it has variable layer heights). Cura has an adaptive infill that creates denser infill around walls and stuff, but leaves the center parts more hollow. I find that particularly cool because it makes sense, most of an object's strength comes from it's outer shell. Dense infill in the center of objects doesn't help much for the overall strength. Things like bones use this type of structure to keep a good strength to weight ratio.

PVA works fine as a support material. No it's not a super strong structural material but it doesn't have to be. The 'spongier' it is, the easier it will be to remove.