I think what you're talking about actually happened in the Insurrection arc back in Suramar, not in any of the 7.3.5 content with Alleria and the Sunwell. The warm reception of the Blood Elves as compared to the taciturn one of the Night Elves was what sealed the deal. In 7.3.5 the alliance between the Nightborne and the Horde is all but a done deal - Alleria's interlude with the Sunwell isn't really a factor and it doesn't make or break Thalyssra's decision. Tyrande's reaction to the Nightborne, while not that stellar for the purposes of the Alliance, is a genuine reaction from a fallible individual. She holds some animus for the Nightborne because she was at the War of the Ancients when the denizens Suramar elected to hide as opposed to fighting with the rest of the Night Elves - Liadrin and Rommath weren't there, and know only of the myth of the War of the Ancients. For Tyrande it was real - people died and the world was shattered while the Nightborne hid themselves away like frightened children (from her perspective), her coldness might not have been diplomatic but it was certainly justified.
I don't think the Alliance are incompetent at all in the this scenario, and I think it's also telling that they also don't seek out the Nightborne for an alliance in the first place (at least not in 7.3.5). Insofar as we know it's not even brought up, so it's possible the Alliance considers the Nightborne not worth pursuing in the first place - possible because of Tyrande's view of them or possibly because they're aware the Horde already have an "in" given Thalyssra and Silgryn's rather o
Well, better is always an option - but it's as much a matter of iteration as it is forethought and planning. What I mean is that the format of an MMORPG is already a limiting factor when it comes to storytelling, and you would get much more detail and flourish if the story were delivered in a book or in a movie. But the mistakes you're referring to are present already in the story - they're part of the Insurrection arc, primarily in the person of a biased diplomat like Tyrande and a rash partisan like Vereesa. I also don't think this is a matter of the Alliance leaders all convening and deciding as a group to elect Tyrande as diplomat - so the blame, if you want to assign it, probably doesn't sit on Anduin's shoulders. Tyrande was in the area of the Broken Shore because of Malfurion being kidnapped, and she no doubt inserted herself into the Suramar affair by virtue of proximity and connection to the land's historic significance (to her). The same is true of the Blood Elves and their presence - I doubt Sylvanas or Saurfang were involved with their insertion into the Nightfallen Insurrection, either - it was a unilateral move that luckily goes on to bear fruit for the Horde.
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I don't think the Blood Oath of the Horde is to a specific individual, it's to the office of Warchief which is a mobile and inheritable position. It passed from Thrall to Garrosh, from Garrosh to Vol'jin, and now from Vol'jin to Sylvanas but it still binds those loyal to the Horde to their Warchief. You can certainly break the oath and turn from the Warchief but in doing so you would become an exile at best, or an enemy at worst.