Emma is mistress of Hartfield (from chapter 1) - not Miss Taylor. Now Mrs Weston is mistress of her own house. I understand that while legally the husband in Regency England had nearly all of the rights, it was normally the custom for the wife (or oldest daughter still at home) to run the household. Both Emma and Mr Knightley appear to expect that Mrs Weston will do this.

Mr Knightley's reference to to be secure of a comfortable provision may mean that in the marriage settlement Mr Weston settled a certain sum of his money on his wife in the event of her surviving him.

To me, the word "obtain" doesn't necessarily mean that one has obtained a commodity, and sometimes the word "obtain" is used in contexts that have nothing to do with commodities. For example from Shakespeare, (by way of http://www.selfknowledge.com/65057.htm), "By guileful fair words peace may be obtained." Peace is not normally regarded as a commodity. In Johnson's Dictionary the definition of obtain is: To Obtain: To gain; To acquire; to procure; to impetrate; to gain by the concession (page 500).