My view on the Darkness/Weather question is to avoid a fair amount of it. Pretty much anything which screws up movement will screw up to-hit modifiers as well. And a penalty to hit will hurt us worse than it hurts them, since they're starting lower on the dice curve. Weather penalizes the lesser-skilled MechWarriors, and we are the lesser-skilled MechWarriors. Assuming a +2 weapons fire penalty, then moving our average TN to hit from a 7 to a 9 is way, WAY worse than their moving from a 5 to a 7. And remember, the Clanners are much more adept at using Reckless Movement than we are due to their lower Piloting Rolls; even in Darkness they can still choose to move full speed and they've got a decent chance of pulling it off given all the 2 and 3-rated pilots.
If we want to slow them down, then we should be angling for Weather - specifically, trying to get Fog penalties. Fog doesn't really affect weapons fire, but it WILL affect movement, and in theory, our movement matters less than their movement (objectives aside, we're already occupying a position whereas they have to attack it), so adding +1 MP per hex hurts them much more than it hurts us, and I don't THINK you can use Reckless Movement through Fog (IIRC that's a Darkness-only thing, but I could be wrong).
We also don't know where their Reinforcements are coming in, which means that if we get caught out of position to intercept them, their Reinforcement group(s) could get a clean run to our board edge if we can't reposition due to enviro conditions.