I've been thinking...
Perhaps this can be incorporated into an NPC's combat AI: They will avoid toxic gas clouds and acid puddles if there is an alternate path available. Take the Zone Rats, for example: I hate it when they run through the toxic gas shroud at the derelict warehouse which is very thick in concentration, or if they return to their original positions and stand there even if there's a pool of acid at the very spot where they are standing. The latter, I think, can be solved with this: "If there is an acid puddle at their original position, they'll stand as close as they can to that position until the puddle completely dissipates, after which they will stand at their original spot once more"
So, in Mordre's case, I reckon that a possible solution to this scenario can be:
If there's a toxic gas cloud that is caused by the player's gas grenade around/near him, he'll walk away from it if he's within the cloud where he'll take damage, and he'll turn hostile to the player, much like how the denizens of SGS's interior turn hostile to the player if they take damage from their gas grenades (mentioned in a previous post in this thread, haven't tested that myself), and changing the Mushroom Cove to a "Partially Controlled Zone".
*I recall in a previous version, the Mushroom Cove was a partially controlled zone. In the current version, it is an uncontrolled zone. Despite being an uncontrolled zone, attacking Mordre directly will turn South Gaters hostile, as after turning him hostile and exiting Mushroom Cove one will find the SGS Sentry at the entrance to Mushroom Cove hostile. Passing the 90 Persuasion check due to circumstances of a certain questline, however, will give the player the option to fight him without turning SGS hostile. (Tested this part myself)
Now, in most cases, uncontrolled zones mean that it's a place where 'whatever happens there, stays there, the news doesn't spread to the faction'. Even though it is not directly mentioned in the zone control's tooltip description. Perhaps stating it there can give future players a better idea about this rule.