And yet you are missing all the political nuance and the reason why we have thalassians on both Horde and Alliance; the whole reason there is a need of Auric as a high elf representative in Sunwell Plateau is because thalassians are not unified politically, the whole reason for an ambassador is to allow those who don't want to join the Blood Elves to still have access to the Sunwell.
You might have not recognized the implications, but I think you have been made aware of them thoroughly by now.
The fact that Auric was presented as not rejoining the Blood Elves and instead acting as a liaison to allow pilgrimage to non-Horde elves certainly indicates he puts equal access to the Sunwell above other political considerations. And again, is not like he couldn't change his mind, but it would certainly bee seen as a betrayal of is initial intent.
Which again brings me back to "Why Auric?" when it could be a newly introduced character.
And really, is not because "Auric is Alliance we can't give him to the Horde" but that his character is evidently more neutral leaning when his motivation is allowing all high elves to have access to the Sunwell. His motivations are outside the Horde/Alliance binary, so again, to make him Horde all of the sudden, while not impossible, it's certainly a big twist from his previous characterization.
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Honestly it could be Auric, but it would have to be in service of an end to the elven pilgrimage to the Sunwell, cause the moment he is found out, it is just over.
Given how yet another group of thalassians has left Quel'thalas -void elves- it could very well lead to the Blood Elves being far more restrictive, and I would see them as far more careful to not alienate more of their people, which could lead to the very idea of pilgrimage to be considered too subversive to continue. For good or ill, the BElves withholding access to the Sunwell for both safety concerns and political reasoning is entirely plausible, specially when they are ostensibly allowing visitation to suversive high elf elements.
The whole political situation of the Sunwell should be very complex now, so it might be a good way to put an end to it one way or another, which would further deteriorate relationship between horde and alliance thalassians.
And tbh, Auric could fill whatever role -he could certainly flip to the Horde, it's believable if he is threatened with loosing access to the Sunwell- and while I do see him as ostensibly neutral, it seems to be by necessity, because he is indeed introduced as an Alliance character, and Auric's Angels are quite likely named after him; he is quite literally the most known Auric in WoW, -it really wouldn't make sense for a group to reference another Auric- so his alliance sympathies are well known and present.
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IMO the best option for that type of narrative -High Elven pilgrims that have decided to rejoin the Blood Elves- would be to simply introduce a new character that is a pilgrim that decided to rejoin the horde and seeks to convince more elves. I don't think anyone would be against a new character in that role.
Yet in any case,
Thalo'thas Brightsun is already a very pro-horde and anti-alliance High Elf. If we need an already established HE character to act as a subversive of the pilgrims, it could be perfectly be him.