How about this then....If the attack "missses" but is within the defenders armour DB then the bolt/ball explodes on the defenders armour for no damage. If the miss is greater than the armour bonus then a roll is made for direction/distance etc.
And spells that could be cast to help would include Deflect Spell or Deflect.
For the Deflect Spell - My error, I was only looking in the Core Book. Deflect Spell would definitely also be an option.
For the alternate interpretation of DB, since we're outside of RAW, there is no reason not to define the sequencing for DB.
First impact to the Attack Roll - Situational Modifiers
Second - Quickness aspect of DB
Third - Parry
Fourth - Shield (and Cover)
Fifth - Armor
If you consider the attack adjustments in that way, if the attack is within the Armor portion of the DB to hitting, then you can assume the armor was hit, but the attack did not penetrate. For a Firebolt, it struck and flared, but did not hurt the wearer. Perhaps the cloak, cape, or other clothing was singed..... No random damage is possible. For Firebolt I would not bother with determining direction/distance, etc. Simply assume the bolt went where it was intended, but the target either got out of the way, or got hit. If the attack did not hit the target, then assume it continued right past him and the only possible target is someone standing directly behind the target.