Hey,
I'll answer 1. and 2. (3 and 4 is for SniperHF who got through all these things):
1. Larian uses Event triggers to define status areas. The effect gets applied via character script as status influence (permanent status with certain strength until completely removed by script or temporarily replaced by a contrary status with higher status strength; will be re-applied after resurrection). Look at Hunter's Edge as an example:
There is the huge 'DAF_VIllageRain' event trigger that throws the event 'RainEventOn' on a character that enters the trigger and the event 'RainEventOff' when he leaves (you fill in the event names in the sidebar of the trigger). The two events are catched in the 'Base' character script:
EVENT RainEnter
ON
OnTriggerEnter(__Me,"RainEventOn")
ACTIONS
AddStatusInfluence(__Me, WET, 20)
EVENT RainLeave
ON
OnTriggerLeave(__Me,"RainEventOff")
ACTIONS
RemoveStatusInfluence(__Me, WET, 20)
So the character gets the WET status with a strength of 20 (strength can sum up). Every house of Hunter's Edge has another event trigger ('DAF_NoRain1', 'DAF_NoRain2'...) that throws the events 'NoRainEventOn' (trigger enter) and 'NoRainEventOff' (trigger leave).
The character script adds the status influence WARM (same strength):
EVENT NoRainEnter
ON
OnTriggerEnter(__Me,"NoRainEventOn")
ACTIONS
AddStatusInfluence(__Me, WARM, 20)
EVENT NoRainLeave
ON
OnTriggerLeave(__Me,"NoRainEventOff")
ACTIONS
RemoveStatusInfluence(__Me, WARM, 20)
The reason for not just removing the WET status influence is that you don't leave the WET trigger. Warm and wet influence 20 will result in zero status (I noticed a short warm period when entering a house sometimes; it doesn't seem to be 100% accurate here). When you leave the no rain trigger (i. e. the house) the warm influence gets removed, so you have WET 20 vs WARM 0 again.
There's a location with three fireplaces in Hunter's Edge; they have the 'PUZZLE_StatusApplier' that adds another 20 warm strength in a radius of 10. So only when you stand in the range of several fireplaces you'll get the warm status (40 or 60 warm vs 20 wet), otherwise it's par to par (20 vs 20).
That's how it works. It would be great if we could add a status influence to an atmosphere (not just a visual effect). But currently there is no actual weather system in D:OS.
2. In the sidebar you can fill in equipment. There's no script command to let non-global characters equip a certain weapon, as far as I know. There's only 'CharacterSwitchWeaponType' but it seems to have hardcoded conditions (range? AP?) and doesn't allow to define a specific weapon from inventory.
