The rules are actually pretty straightforward:
2013 Tournament Pack wrote:
A unit must use their counter charge move to move directly
towards the closest enemy unit. It may move into base contact if
close enough, and as long as the enemy is not already in contact
with two defending units. Units can choose not to counter
charge if they wish, but if they do counter charge they must
head towards the nearest enemy.
The FAQ gives one slight modification of this, in that if the nearest unit is "fully engaged" (i.e. contacted by two of your units) you can proceed towards the next closest enemy unit.
To me, these rules are very strict about what and where you can counter charge, but it seemed to me that not everybody played this way. Sometimes a strict ruling would lead to a congestion of units where only the front units would make it into base contact simply because a path "directly towards" the closest enemy unit (or next closest if the closest is fully engaged) would lead a unit to simply hit his friends in the back. Most players (including me) would, I think, agree that a slight detour is OK here, but I'm not sure if the rules actually allow that. I had one situation with 6 Shining Spears vs 6 Warp Spiders (first round draw, then counter charge) were the Spears wanted to surround the Spiders and contact them all, but I argued that the first four Spears would have to contact the two Spiders in front, leaving the Exarchs (unit 5 and 6) free to use their FF value. Reading the rules I believe I was correct.
A more uncertain case is one I observed where a Revenant (war engine) wanted to counter charge two Monoliths (it was engaged by a Warrior Phalanx). If it moved in a straight line it would contact both monoliths, but would then be out of range of one of the Warrior units. I argued that it had to charge straight towards the middle of the closest monolith, and then use Barging to continue straight towards the second Monolith. This would keep all the Warriors in range.
1) Am I correct in that "directly towards" is this strict? Should it have been towards the closest point of the closest Monolith? Or is it enough that the unit that counter charges moves toward any part of the target enemy unit?
2) Could the Revenant have continued its counter charge towards a third unit if it had any remaining move? (Not relevant in this case - it would have been back towards the Warriors.)