Bart de Groot
Can an artillery unit fire at non artillery units at double range? All the rules seems to be about firing at artillery pieces at double range...
No. Attacking Double Range Artillery can only attack an enemy artillery unit.
Also can artillery shoot at as many units as are at double range? What if there are 5 targets surrounding an artillery at double range because they all had to retreat the prior turn? Is that ok?
If there are 5 enemy artillery units that are valid targets for your attacking double range artillery you may engage all 5, but this is a single combat against all 5 combined, not 5 combats (you may not like the odds).
What happens if its target are stacked hexes with artillery and infantry? Does one add up the defense value of the infantry unit? Assuming the artillery fire succeeded at double range, which unit takes the damage (art or inf)?
If the enemy artillery unit is not engaged in normal combat it can be the target of a double range artillery attack, only the artillery unit defends and would take the damage if any.