No. If any of the charging units are too close to the charged unit for it to make a Stand & Shoot charge reaction, it cannot Stand & Shoot.

When the charge reaction is declared.

No. As stated on page 143, models that know such spells can attempt to cast them when chosen during the active player's Shooting phase. Perhaps obviously, this includes Bound spells.

No. A model cannot shoot during the Shooting phase if it marched during the preceding Movement phase and, although they work differently to most missile weapons, Breath weapons are a type of missile weapon.

Yes.

No. To claim the benefits of being on a hill, a unit must be entirely on the hill.

A unit that is closer to the top of a hill (i.e., the centre of the hill or the edge of the battlefield) can draw a line of sight across and shoot over one that is closer to the bottom. Units cannot draw a line of sight across or shoot over units that are closer to the top of a hill than they are.

Missile weapons mounted on chariots (or howdahs) are shot by the crew, using their Ballistic Skill.