] ] 1) Had JA in her mind a real portrait when she wrote the novel? I know from one of her letters quoted in this same board that she visited collections seeking for paintings that looked like the characters in her books. Maybe she didn’t find her Darcy but this shows that she knew the art and the artists of her epoch.

I think I can answer this question. About the others, I would not even try to find an answer, as I am no art expert.
About the collections, Recently I've read a biography of JA (by Valerie Grosvenor Myer ) and I can remember that she says that one time, after JA wrote (and perhaps published) Pride and Prejudice, she went to a art-exhibition with some friends. JA looked for painted potraits of her characters, while she was visiting that exhibition. She said that she couldn't find a good portrait of Darcy, but did find one who had some simularities with lizzy.
I think we can conclude then, that JA didn't base her image of Dacry on a excisting portrait.