Well I've been doing a little reading and it would seem we haven't quite learned all the rules yet for Kings of War.
There are two ways that a unit can become disrupted. It can take melee damage or it can become wavering. If a unit takes melee damage it automatically becomes disrupted. If a unit becomes wavering it is also disrupted. When a unit takes ranged or magic damage it does not become dsirupted unless it becomes wavering from the nerve test.
Regarding hindered charges ... all charges are hindered regardless of how long the unit has been in hindering terrain. Obviously Pathfinder and Strider override this. All counter charges are never hindered by anything. So if unit A goes and sits in rough ground and unit B charges unit A then unit B's charge is hindered. Unit A's counter charge is not hindered because it's a counter charge. So it's not really the first round of combat for each side. It's just the initial charge.