To answer Don's questions:
If a character is added to a chariot his characteristics and that of the chariot remain the same. Although he replaces one of the crewmen in reality, the effect this would have on the chariot's stats are ignored in order to keep things simple.
Egyptian shooting attacks are treated as shooting attacks, so don't effect the result of the close combat unless a) they cause a panic test be inflicting 25% plus casualties, or b) they cause enough casualties to reduce the targets rank bonus.
Finally, challenges are fought using the normal rules, apart from the changes specifically listed in the rules section on challenges. So chargers strike first, unless special weapon rules apply, etc.
Jervis