1) Yes, with proper compaction reports.
2)I would rather buy an older house on fill than a newer one and do due diligence as you are.
3)Common in SD county.
4)It depends. If the entire yard has steep areas, I don’t like that. If given a choice I would chose to build a stepped foundation over a slab on fill.
I have seen a pad built where some native soil should have been removed and better fill brought in and it was not done. Maybe you could dig around the footing, especially on the low side and check for type of soil. Good fill can compact as hard as undisturbed soil that is suitable to build on.
Many areas of SD have what is called “expansive soil”, Check into that.
