This game doesn't necessarily describe different effects you might find on your equipment or perk cards. Things that work one way do something different upon interacting with particular legendary effect or craftable mod. The only way to assess actual effectiveness of given element is to test it in practice.
Dodgy - it seems that it calculates the damage to be reduced based on "sent damage" (before reduction from armor etc), then reduces it from "applied damage" (the damage after all protective effects). As a result, quite often it negates not 30% of the damage for 30 AP, but instead 100% of the damage for 30 AP, effectively turning your AP bar into another HP bar. This perk is critical for PvP.
It's not obvious what different perk cards mean when it comes to weapons being explosive. To clear this out: Heavy gunner perks work and boost damage of non-explosive heavy weapons (this is a closed list of weapons), even if these weapons have a legendary modifier that makes their bullets explosive. Bullets with explosive legendary mod deal two sources of damages: the base damage and the explosive Area-of-Effect damage. Demolition expert will not boost base damage, but will boost the AoE effect. 5* in Demo Expert translates to 60% increase of 20% buff, hence, its overall a 12% to how much damage you deal per shot, 2.4% boost per perk card. Quite often other perk cards would give more advantages tho. Explosion-radius perks
"In combat" - Regenerating Armor (currently overnerfed), Healing Factor mutation, Stalkers Weapons, Sneak Shot mechanics - they all depend upon the status of being "In combat". There are two main checks for this mechanic - first, if there's any monster in the game that's currently hating you (and you in particular), and second, if there's an opponent player within 10 meters distance from you. If there's none - no monster, no opponent player, you're not in combat. The fact that you're being shot by an opponent player that stands 15 meters from you mean nothing, you're not in combat, you're regenerating health