Regading your question 2, I believe Anna is the daughter of James Austen by his first wife, Anne Matthew.
Jane Anna Elizabeth Austen (1795-1872) on marriage become Mrs Benjamin Lefroy.

I also find the meaning of Jane's quote rather obscure. I may be wrong, but my impression is Jane is not well pleased with her niece Anna's second engagement, this time to Benjamin Lefroy.
Apparently, Jane felt abit nervous of Anna's behaviour and what she might do next, if not with one young man, then with another-.
'she is at about her third or fourth, which are generally simple and pretty' (?)

Anna was an attractive young woman, considered by the family ,apparently, to be unpredictable, wayward and unsteady in temperment until her marriage to Ben Lefroy.

Hope this is of some help. Admittedly, the first part of the quote quite eludes me.

With reference to C. Tomalin. 'Jane Austen. A Life'.
Penguin Books. 2000. Ch. 20