In (6), if you said the rice it would mean the rice she had bought specifically for the occasion. Without the article it could have been any rice. The text has already told us that she bought specific things for the occasion.
For (8) I think your book is wrong. My reasoning is the same us above, The passage describes Mrs Midori's preparations for her dinner party. It tells us that she bought chicken. Therefore, when the passage subsequently refers to chicken, it means that chicken, the chicken she bought,
If you said the second pot your sentence would become unnatural because the passage has already stated that there were many dishes being prepared. Therefore there must have been many other pots, and the indicates one specific alternative, as I said above,
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