Let’s use the available sources that we have to determine if Quel’thalas current political position is gameplay driven only or not.
Warcraft 3 set the story in such a position that pretty much there were only Blood Elves left and not even a single High Elf. There was no determining the splintering of the elven people in that game.
It was WoW that showed this division. And it really had a gameplay roots.
Quel’lithien, Quel’danil and Farstrider Lodges were set in Classic WoW. Back when RPG was still canon. And already RPG introduced Blood Elves as lost people dabbling with dark magic that made them hostile to both Alliance and the Horde with only Goblins remaining as the sole ones willing to trade with them.
So that’s why High Elves were introduced as bunch of refugees. Their state was so miserable that one could compare them to homeless rats.
With one short story about a lone High Elf in Theramore Inn being bullied by two Dwarves who stole his dagger. The only memento of his homeland and family upon which the Elf used his magic to snare Dwarves, take back his dagger and flee.
The three Lodges placed in Plaguelands, Hinterlands and Loch Modan were designed to serve as hubs for the High Elven faction called “Silvermoon Remnant”. It was supposed to be an Alliance friendly reputation group. Something a kin to Silver Covenant.
But it never came to be, because at that time Blizzard was changing the lore and focused on the Blood Elves. Decided to make them less Diabolical. And so from people lost in dark arts they became grim, pragmatic survivors. And thus the three Lodges full of High Elves lost any real significance and were just left there.
But Blizzard didn’t want to fak up the existing lore too much since they already scored few mistake gems with one such being where Chris Metzen himself explained that he simply forgot about the origin of Eredar when he related them to Draenei.
Although old lore was already in motion and couldn’t be reversed immidiately and that is why we were introduced to Kael’thas and his Sunfury that was the embodiment of those “evil” Blood Elves.
High Elves on that matter were turned in McGuffins serving as an excuse for the Alliance to mess with Thalassian lore, such as Quel’delar and later Zul’aman. Otherwise Alliance would have no reason to even care about those things.
So yes. Gameplay reasons in that matter are big. And may even be the primary case why Blood Elves and High Elves ended the way they did.
Now let’s focus on the Lore aspect.
Warcraft Chronicle vol. 3 describes citizens of Quel’thalas as a nation of isolationists who care little about the outside world. They just wanted to be left alone to their devices and make contact with other races only when necessary.
So your standard High Elf living in Silvermoon didn’t give a sh!t about Alliance or the Horde. Even when the rest of the Eastern Kingdoms were at war, with Kingdom of Stormwind falling, Dwarven Kingdoms being in retreat and Humans forming desperate coalition to face the threat.
This obliviousness is what later became their doom. Anasterian sent token force to aid the Alliance during Second War only because his bloodline was indebted to that of Anduin Lothar’s due to the fact that his ancestor Arathor himself helped Elves to deal with Trolls in times ancient which resulted with Thalassians teaching Humans some magic.
Once the threat was dealt with, Dark Portal closed by Khadgar and Orcs sent to internment camps, Thalassian indifference once again kicked in. Like truly if Americans have Thanksgiving Day. Then Thalassians should have Fooksnogiving Day.
Anasterian officialy pulled Quel’thalas out of the Alliance and ordered every High Elf remaining in it’s ranks to go home.
And this is where Warcraft Ultimate Visual Guide from 2013 kicks in that describes this event.
Some Elves returned. But not all. Others who formed bonds of friendship and brotherhood in arms with Humans and Dwarves refused to return. They outright objected the royal decree of Anasterian Sunstrider.
The best example of it would be Vereesa Windrunner who at that time was enamored with her Human later be husband Rhonin.
So no. Being loyal to the Alliance is not a High Elven thing. Only the tiny portion of them who were exceptions to the rule stayed behind. Statistic High Elf didn’t give a crap about Alliance.
This of course left the Alliance High Elves disgruntled believing that one day such point of view might bite Thalassians in the a$$. And boy if they were not right about that.
It is worth noting that Kael’thas at that time also was an exception to the rule. Crown Prince who preferred other races than his own people. Living among them in a distant land. Thalassains couldn’t understand what driven him to do so. And thus never could truly connect with him.
This desire of his even worried Anasterian himself who could only hope that one day his son will become fed up with this style of life and embrace his heritage.
Arthas’ invasion was a major blow back in the Third War. But what could be noted is that High Elven defenders with Sylvanas in front called him. “Human traitor Prince” for example.
Which indicated they blamed him not just as an undead, but as a Human in general. A Human that coused greater harm and destruction than any Orc ever did. Arthas was the embodiment of that what is the worst and darkest in Human heart.
Orcish Horde never made it to Silvermoon. So it’s citizens didn’t suffer Horde’s treatment. But they did suffer from Human Arthas’ hands.
Which is why Garithos who on his own would be quite insignificant, became such a pain.
Again we go to Warcraft Chronicle Vol. 3.
When Kael’thas came to the ravaged Kingdom of Quel’thalas, his people greeted him with concealed resentment. Blaming him for not being there when they needed him the most.
And he only added insult to the injury when he gathered an army and left to join the Alliance. Once again the Prince left to fight a war in a distant land. And what good came from pledging himself to the Alliance? Being shunned as a cheap fodder for the undead.
This loyalty nearly cost him his life and evaporated any desire to serve within it. This was the moment when Kael’thas’ mindset was finally tuning in toward his people.
Garithos was just an another Human that sought Elven demise. And enough was enough.
This was the moment when Alliance loyal High Elf Prince became fed up with it’s current bullsh!t. And with Garithos declaring his execution and all his subjects, any Alliance High Elf should have adopted Blood Elf mantle and stand with his Prince.
But they didn’t. I don’t know HOW in the world the likes of Vereesa avoided Garithos’ wrath but somehow they did, hidden within Kirin Tor, equally oblivious to see their Liege, their Prince to be killed unjustly.
We know that Grand Magus Telestra, a High Elf herself and Kael’thas’ mentor abandonned him when he allied himself wit the Naga, but held a massive grudge against Kirin Tor for standing idle when Garithos ordered mass execution of the Elven kind. Which is why she and her High Elves joined Malygos during Nexus war. She and all that followed her perished.
And they were pure Kirin Tor High Elves which makes me wonder what Silver Covenant was made from? Where did those die hard Alliance supporters came from? Theramore? Allerian Stronghold?
The point is that Blood Elves joined the Horde because Sylvanas Windrunner who died defending her Homeland in life, still desired to do so in death. And it was she, a dead hero of her people that extended the hand and made the offer, serving later as mediator between Lor’themar and Thrall.