They should let us open them for Halloween- Crux Crystals Inferno and Aqua -50% plunder curse 10 throwing lanterns etc, you get the drift
Mind bards, ego hooks, psyche traps, persona snares combine into doninance nodes. A certain amount of dominance nodes are required for a certain tier of rewards. 50,100,250,1000,2500,5000. Top 100, top 10 Top right hand corner there is a "Chest" icon, click that to see the rewards and details of the event.
Oh no, don't ! You certainly wouldn't want to maximize your HL (Tier 5) buildings (lvl4) as it has the worst stats to price ratio... Abyss is certainly the way to go as it will give you great stats per gold spent. Sure, you'll have to factor the land prices in at some time, but plunderwise new lands with a level 1 building pay better than upgrades.
Defensive buildings will give you slightly more plunder as they contribute 5% more to your plunder compared to the other troop buildings. Nothing exciting if it's only one building out of 100, but I guess it would sum up if you have a lot of them (well, up to 5%...).