The Answer for since this is the only thing I knew in the lore
1] The Remaining Large Population of the High Elves followed Prince Kaelthas after the destruction of Silvermoon, they manage to find Illidan [all thanks to Vashj] and to use demonic magic as there new primary source of use.
while in the eastern kingdoms, the high elves who never left the alliance [the elves that weren't in the domain of Lord Garithos, but of the breaking of the alliance of elves and humans after the second war] probably was taken refuge by Jaina proudmoore I think.
2] yes she got resurrected in a form of a banshee in wc3, but since blizzard wanted the game more epic, they decided that Sylvanas wasn't killed by Arthas, rather she was killed by Arthas in a different way.
3] Actually the Dreadlords aren't the field of evil, there are countless more demons that infiltrated Sargeras and made him mad, there are only called Primary because of there ability that shook Sargeras' mind even more.
4] Not all Humans follow the light, there are actually cults that adored undead living like the acolytes, some I don't know, and based from World of Warcraft, humans cannot become druids, only ancient ones like night elves, taurens, draeneis.
5] They Actually serve the Scourge during The Lich King's rule but after his death, they lose control of his mind powers and they became mercenaries in the game.
6] I don't know but since Eredars are actually Draeneis that lived in Argus, maybe there are some lurking in the wilds
note that I only answered this because I only played w3 not wow, the rest I just read them in books, internet, and to smart ass players
