I could see this going either way. As you note, Kael’thas had to return home to deal with the aftermath of the Scourge invasion, as he, ironically enough in retrospect, had been one of the Quel’dorei who maintained his connections and obligations to the Alliance and the world outside Quel’Thalas after his father removed Quel’Thalas from the Alliance. Yet, in Shadowlands he tells both Sin’dorei and Ren’dorei players that “Nine of every ten of us,” were slain by Arthas in the invasion, including himself in the 10% that lived, even though he was not present for the attack. That would seem to imply that the 90% death rate statistic was across all of the Quel’dorei on Azeroth, not just those in Quel’Thalas at the time of the invasion. I would not, however, include the Quel’dorei trapped in Outland in that statistic, since they would have already been assumed lost at that point.
Overall, though, I don’t think the specific details matter that much. The broad point is that the Scourge invasion wiped out the vast majority of Quel’dorei, and of those that survived the vast majority adopted the name Sin’dorei. Personally, I’d argue that this means there’s a real possibility that the overall majority of Thalassian elves still walking around on Azeroth are undead, existing as Forsaken, Ebon Blade, or Scourge, with the Sin’dorei being the second largest group and the largest playable faction of Thalassian elves.
This gets tricky though, because that group of anti-isolationist Alliance-aligned Quel’dorei that broke away from Quel’Thalas pre-Third War includes both Kael’thas and Rommath, who held high-ranking positions in Dalaran. We don’t know how many of those returned home with Kael’thas and took up the Sin’dorei name, or broke away from the Alliance because of Garithos, but we know that there were some.
His dialogue there is one of my favorite things in Shadowlands, though the line I like most is only available when the player isn’t playing a Blood/Void Elf: “You still bleed, do you not? Cut your flesh and count the drops of blood. Let nine fall and soak the earth for every one you save. Tell me what that does to your heart, -player character name-. For I can tell you precisely what it did to mine.”
The 90% were killed directly by the Scourge, the withdrawal symptoms weren’t apparent until after the corrupted Sunwell was destroyed. That’s why the need to find a cure or management solution was so vital; 90% of the elves had already been killed, and then in addition to that survivors who were very young or very old started dying from withdrawals as time passed. With their youngest children unable to adapt to the loss of the Sunwell, extinction was a very real possibilty, and it’s that fear that explains both Kael’thas’ desperation to find a cure and Lor’themar’s exile (but not execution) of those who shunned the mana tapping of living creatures.
Something that I think should be kept in mind is that the reason Halduron asked Vereesa for assistance wasn’t because Silvermoon didn’t have enough Farstriders in general, but that most of them were out on assignment and could not be recalled easily on short notice. It is noteworthy that Vereesa could rally enough rangers to be worth contacting, but this instance doesn’t really give us any idea of how big the Silver Covenant is compared to the Farstriders. I know that’s not necessarily what you were getting at, but I thought it worth adding to the general conversation.
That was specifically the Silver Covenant, not a generic group of Alliance Quel’dorei, and Vereesa had already shown her willingness to insert herself into anything involving Sin’dorei goals. She would have been there even if she only had a dozen elves backing her up. Don’t get me wrong, it’s noteworthy that they were there at all, but I wouldn’t take that instance as indicative of their overall numbers. It’s not like the Silver Covenant has a lot to do aside from go where Vereesa tells them, they’re not tasked with also protecting a nation like the Farstriders are.
Yep, and if you’re playing a Blood Elf hunter she has extra trash talk for you beyond what she says to other Horde players. I actually like that she has different quest text in this instance, but the rest of the order hall does make me wonder why she makes remarks about not being able to stand the Sin’dorei leadership and then works with Halduron without any sign of animosity. It feels like I’m missing some sort of backstory there.