- What percentage of regular damage is a counterattack?
I suspect that counterattacks deal damage computed using half the attack score of the unit making the counterattack, rather than by halving the damage which results from the full attack score. In the case of a counterattack against a zero defense target, this results in half damage. Against a target with some amount of defense, the reduction in damage can be much greater.
Regardless, the XML file CoreUnitStats.XML includes a multiplier for counterattacks, which is listed as defaulting to 0.5. Thus, the percentage reduction for anything that lacks a counterattack damage multiplier should be 50%, though this is not necessarily a 50% reduction in damage (again, I suspect the multiplier is applied to the attack score rather than to the damage).
- What percentage of of damage is the swarm bonus?
In case you don't want to read what Primal_Savage linked you to, the swarm bonus is not a percentage bonus but rather an additive attack and accuracy bonus (1 attack and 5 accuracy per friendly unit adjacent to the target). Thus, the percentage damage bonus granted by swarming an enemy is highly variable, depending on what your units' attack scores look like, on what your target's defense score looks like, and on how likely your units are to hit their target.