Lastly, how does the poisonous berries appear in the novel and what do the represent?

The berries represent more than simply taunting President Snow at the end of the novel. They become a part of Katniss's identity.

The berries. I realize the answer to who I am lies in that handful of poisonous fruit. If I held them out to save Peeta because I knew I would be shunned if I came back without him, then I am despicable. If I held them out because I loved him, I am still self-centered, although forgivable. But if I held them out to defy the Capitol, I am someone of worth. The trouble is, I don't know exactly what was going on inside me at that moment. (ch 8)