1. Do Accidentals override key signature and previous Accidentals? 2

2. The purpose of the key signature is to minimize the number of such accidentals required to notate the music.

3. Accidentals in the Key Signature can be changed temporarily by another Accidental and are canceled by a Bar Line

4. - Acciaccaturas and groups of grace notes - Dynamics, fermata - Tie individual notes or whole chords - Change clef, time and key signature at start or in middle of score - Repeat bars, DC, Segno etc

5. It does not discriminate between a major key and its relative minor; the piece may modulate to a different key; if the modulation is brief, it may not involve a change of key signature, being indicated instead with accidentals.

6. The reason you see double Accidentals so rarely is that a D raised by two semitones is enharmonically equivalent to (in other words, it sounds the same as) an E♮, which is in the key signature of C Major.