I just tested this behavior by using same testing scenario both in AOC and DE. IMO units behaved pretty similarly in both versions of the game. When I selected single unit and right-clicked him on some enemy unit in group battle, then he often instead choose to attack closer unit.
This behavior might seem a little bit unintuitive, but it is needed in order to keep right-click working properly when ordering units to attack group of enemy units. Units must be able to attack closer units in order for group attack to work without units overprioritizing reaching single targeted enemy unit. This was broken for quite a long time, but thankfully was recently fixed.
Could you please check right-clicked units behavior in AOC and compare it to behavior in DE? Are there any significant differences for you? AOC behavior should generally be kept so that gameplay generally remains the same as has been for 20 years.
Yes, that method is often used and is well-accepted solution to the problem of attacking single certain unit in group battle. I believe that it would be good, if double-right-click would prioritize attacking targeted unit over closer units.
Should definitely be fixed, but appears to be a different issue.