@Travis Baumann Hey buddy, lemme see if I can help you out:
1: Ultimately, to properly understand an army all you will need is the faction's book. The GHB has points for matched play but isn't a requirement (also you can probably find an image of whichever force you want.)
2: Keywords are at the bottom of a units Warscroll. For example, a Tzeentch Lord of Change (Magic blue chicken) has the keywords Chaos, meaning it is faction chaos, Daemon, Tzeentch, Monster, Hero, Wizard, Lord of Change. This means abilities or weapons which effect any of those, such as the Tzaangors banner which works off friendly keyword wizard, would work with the Lord of Change.
3: generally by reading the warscrolls and trying to get your brain around it all. This forum really helps me, some people are great list builders. Battalions will be in the faction's book.
The Dark Aelves are listed in Order as whilst they are malicious, they live in an ordered society of cities. I wouldn't be surprised if we see a bunch in chaos one day with Slaanesh. So the Order book would have what you needed.