If Charles died before succeeding to the throne, I think Camilla would still be known as Duchess of Cornwall. The Duke of Cornwall title would merge with the Crown on Charles's death, but Camilla would still be styled as Duchess. I'm sure she would still do royal duties if she wished to.
If Charles died as King and Camilla survived him, she would still be Queen Camilla.
Widowed consorts are usually still known as Queen [her name].
Queen Mary was still Queen Mary after the death of George V.
Queen Alexandra was still Queen Alexandra after the death of Edward VII.
After the death of George VI, Queen Elizabeth was mostly referred to as the Queen Mother because the present Queen is also named Elizabeth. If Margaret had been the elder and succeeded to the throne, I expect the QM would have carried on being called Queen Elizabeth.