Still, there are high elf necromancers in game as it is, (Inspector Malicia) and several warlocks in different cults around the world. Now, how does this relate to a possible playable faction, It's just about the parameters said faction is granted.
Eeven if I contest the notion of necromancers being accepted into factions (as they are shunned my most races), I see this argumentation in favor or against as superfluous since we have more poignant evidence with the high elf warlocks in game, sanctioned by the biggest high elf organization, the Silver Covenant (the faction that hypothetically has the highest possibility of becoming playable). Warlocks are like one step away from being demon hunters as it is. Demon Hunters and Warlocks are about self sacrifice, and I don't think that any race, except one particularly pious or religious, would be totally against them. High elves are neither of those things.
Of Note; the "high elven druids" was changed to "high elven magi" in the encyclopedia; a much newer and supplementary current lore source for the game. So any ideas of druidism in high elven culture are speculative at least (still I personally believe that rangers used some very limited druidic magics on their line of work, but very minimal and overall negligible. I love Dark Rangers, and whilst I love the idea that they corrupted their druidics powers to call in the shadows, since the RPG's are non-canon, I can only see it as speculation )