I'm confused, what do you want to say ? Of course the NE wouldn't join the Horde, imo they shouldn't be part of the alliance, too. Ne are not highelves. They joined the alliance like everybody else did, to defend Mount Hyjal against Archimonde. But that's not the Alliance meaning, playable faction in WoW today.
... The whole Alliance - Horde thing just doesn't make sense... :/
I'm saying that they had to join someone, and the Alliance was the only option. It was that or deal with the Horde alone. After the temporary alliance and truce to defend Mt. Hyjal, they still had to deal with the fact that the Horde had established a strong foothold on Kalimdor. Their options were to stand and hang alone, stand with the Alliance, or stand with the Horde.
They didn't stand with the Horde because a tribe of orcs killed Cenarius, and while Tyrande and Malfurion might have set that aside, I don't think the entire society would.
The Alliance/Horde thing does make sense in that both factions established themselves from the beginning. The Horde came through the Dark Portal, eventually recruited goblins and forest trolls, were broken, and Thrall built a new Horde. The Alliance formed to defend against the Horde. Of course, the Scourge and the Burning Legion made alliances a necessity, and continuing cold war-level tensions with frequent violent outbreaks made neutrality a difficult, possibly suicidal option.
Uhm, can you say, where it is mentioned that Sylvanas brought them into the Horde. Don't the bloodelves hate everything undead, including one of the former guard captains ? And why should Sylvanas do so ?
The blood elves hate the Scourge, which devastated their homeland. The forsaken - who are undead who have freed themselves from the Scourge - also hate the Scourge, who also devastated
their homeland. The Forsaken are led by Sylvanas, who, among other things, was a high elf who gave her life defending Quel'thalas against Arthas' advance. The Forsaken have offered extensive aid to the blood elves, share the same enemies and many (not all) of the same goals. Most especially, since the blood elves joined the Horde, the forsaken have worked hard to help the blood elves purge their homeland of the Scourge - hence the large number of undead NPCs in Tranquilien.
And I'm not sure if you're saying that Sylvanas was just a guard captain - she was sister to one of the heroes of who went beyond the dark portal (Alleria Windrunner) and was herself the Ranger General of Quel'Thalas. The
Lament of the Highborne quest chain makes it clear that she has not abandoned her feelings for her heritage and life just because she was made undead.
There is no other highpriestress ? Technically Azshara is still queen of the nightelves. I can't see any other queen (or king ?)mentioned. While Malfurion and Tyrande lead the Ne together in RoC, it's somehow an unanswered question how NE society actually evolved ?
Azshara is an exile, ruler of the Naga, formerly one of the Highborne. The night elves wouldn't follow her. There's no confusion anywhere as to whether she could rightfully rule the night elves. Effectively, there was a coup.
Tyrande is the highest ranking priestess of the moon. This is not an ambiguous statement. http://www.wowwiki.com/Tyrande
Tyrande Whisperwind (pronounced Ti-Ran-da) is the high priestess of Elune, Mother Superior of the Sisters of Elune and the official head of the night elf government. Tyrande's mate is Archdruid Stormrage. She can be found in the Temple of the Moon in Darnassus.
You can read about the night elven culture http://www.wowwiki.com/Night_elf