Until proven otherwise it is a fact. Plenty of facts have been proven wrong over time. Facts are the truth as currently understood. The lore does not support high elves existing in 'unknown lands'. So I am afraid I am not going to respect an argument that is essentially 'Blizzard might make more up'. They might, when they do we can discuss it. As they haven't, and high elf lore rather strongly suggests that the scenario you put forward is practically impossible, I discount it.
The Nightborne completely disprove this as they were never bonded to the world tree and created the Nightwell to sustain their addiction. This actually darkened their skin tones and left them even more attuned to the arcane. Elves do not change in the same fashion, it depends on their circumstances. You wish for a completely different set of circumstances to produce a group of elves the same as those from Quel'thalas.
And rapidly shut. The Mag'har were drawn from a scenario so detailed it was used as the basis for an expansion. A really shitty expansion, but still an expansion. The alternate dimension came first, the Mag'har came later. They are not going to have us go spelunking through the warcraft multiverse for the sole reason of giving the Alliance white skinned elves.
After the schism? But that was years ago and as every pro High Elfer knows, something that happened in the past and which is inconvenient doesn't matter anymore. Leaving that small hypocrisy aside, do you have any proof that even supports this has ever happened? Even Valeera stayed ideologically a Blood Elf and didn't hook up with the exiles. Perhaps the protesters wished to change Quel'thalas from within and drew the line at treason?
The process for becoming a Void Elf is being zapped. That is where new Void Elves come from. The process for creating a new high elf exile likely involves a considerable amount of time. There is also the fact that any Blood Elf who was disgusted enough to head over to the Alliance likely already did so at the moment of the schism. Even if one or two has headed over since, that still doesn't change the status of the exiles as a nearly dead, almost extinct group.
In the one corner I have multiple developers and fifteen years of Warcraft lore, and on the other I have you saying they are wrong. You don't exactly have the weight of evidence on your side.
I know you keep citing the Void Elves as if that proved anything and it might if two of the times the population issue was mentioned in regards to the exiles were brought up AFTER Void Elves had been introduced.
The precedents all occurred under specific circumstances and their repetition for the obvious purpose of giving the Alliance white skinned elves would be more than a little ridiculous. As for the later, high elves can clearly reproduce with other high elves, but given the canon statements regarding their population and their canon proclivities towards relationships with humans, they aren't going to form the nucleus of a new independent high elf civilization either.
Because the lands unknown were the unexplored Eastern Kingdoms. I don't believe they mentioned any getting lost in the storm, it was upon landing that their troubles started. That, and because they would never have come to depend on the Sunwell and would have led a separate existence, would mean that even if they did exist, they would not be High Elves and would be something entirely different.