Lady Cary did not want to be anyone's concubine and she was pissed when this guy suggested it. She felt that he had degraded her and was pissed that that is how he saw her. She had to make sure he never asked her that again hence why she opted to trample as well as facesit him. The mistress had earlier teased and denied him as part of her punishment.
Lady Cary wanted to invest her money in a small business and she did not want to use a loan. She wanted to use her own money so as to maximize on the returns and minimize risks. She felt that saving it was the best way to do it and hence why she had to avoid paying rent in order to save more. The mistress therefore seduced her landlord to make this happen.