I need more TIME
We are discussing 2nd edition aren't we?
1) If a unit is in "advance to engage" does it have to move into "engaged" on its following turn or can it withdraw back to column? (we played that it couldn't as the arrows on the battle chart would seem to disallow it)
That's the way I play, but the sideways slide from "Advance to Engage" across to "Engaged" is not a move, just a recognition that the artillery of the defender can no longer fire, and that any charge bonus can be taken this turn and this turn only. So the block, having slide across to "Engaged", can then opt to either fire or move back to it's own end of the column in exactly the same way as applies to every turn in which it starts as engaged.
2) If a unit is "engaged" against an enemy column that contains a mixture of types including infantry, cavalry and artillery can it choose to attack the artillery or does it have to plough through the infantry/cavalry first? (we played yes, because we couldn't see anything in the rulebook to disallow it even though it seemed thematically wrong)
I allow this, but such an attack on artillery would not benefit from the bonus versus unaccompanied artillery. And since the defending artillery is both pinned and unable to fire, it is rarely that an attacker would prefer to attack the artillery.
3) Re-enforcements are chosen at the start of the turn, ignore road & river limits when moving from their location to the battle location, and are then placed in the reserve area at the end of the battle turn and can move into column (or advance to engage if cavalry) in their following turn, and, all correct?