I finally managed to get my CD-ROMs to work.
According to that, Isabella Coming is not in the Payne household.
Alex, Jane and Alex Payne are in a house in Pringleton Village, Borgue, the last house listed on Page 11.
The next household listed, on Page 12, is Borgue Free Manse, where the occupants are the Rev Mr McElder, Free Kirk Minister, and Mrs McElder.
Next, also on Page 12, is another household in the Borgue Free Manse, with only two occupants: Ellen Foster, 23, general servant at manse, born Twynholm, and Isabella Coming, visitor, born Liverpool.