[bbz]

I think people suggested Earthquake because half of the skeletons have 4 hp so or up to 8hp. Also the way to use it is with a single caster defending a province that an enemy is about to attack. Use some pd to distract the enemy then all you need is 1 or 2 E4 casters with 5 earth gems each, cast summon earthpower to get them to E5 then set them to: cast earthquake, retreat.(giving them the two extra gem would make them cast earthquake like an E6 mage and reduce the 300 fatigue from the spell enough so that they can retreat(and not be unconcious because of it) ,also you need high defence to avoid being hit by earthquake that is if your mages dont have enough hit points to survive it)

Answer to the second question, they do it randomly in an area around them also they tend to target less armoured targets I think(that is for legions of steel buff, not sure about the others but it has some tricks i think)
What I usually do is set the thug at the bottom and the rear of the map(or top) and place the mage behind the thug so that thier squares form a rectangle. Also I dont place anything near them so that the mage doesnt make a mistake. Another way to ensure that your thug has quickness is have the mage cast it 2-3 times before battle(at least improves the chances of affecting the thug)

About the first question I'm pretty sure it increases your dominion aswell. So if have dominion 8 and 10 temples you can get up to 10 candles in a friendly province.