If they are, either Regina has a wonderful personality or Usatame has terrible taste because Regina is probably the most unnatractive woman i've seen in my life and she's not even ugly.
Pretty sure Usatame bought the house. Everyone else is renting there. But maybe Regina doesn't have to pay, not sure what she does for a living as I doubt she makes enough from modeling.
They are not, they have been friends for a long time. Usa had a very serious long time boyfriend but they broke up quite a while ago. AFAIK Regina was a cosplay buddy that gradually evolved into doing lewd material with Usa.