Adding blood elves to the Horde was kind of unexpected and it shifted the image of Horde greatly. I can remember blood elves not being accepted warmly at first. They become quickly popular though. As for me, I think it hurted Horde integrity by adding highly civilized and sophisticated race to the primal and wild faction. I don't mean Horde can't have these features, but compare let's say Zandalari and High elves and tell what fits with the image of the Horde more.
Personal biases aside, blood elves are Horde race now. That's a fact. Elves are not faction specific thing anymore. Everybody should accept that. There is no point fighting that. Night elves and blood elves made their way to their perspective factions. Nightborne actually offer very little new to the Horde (it already have aristocratic magic oriented elves), on the other hand, they could open very interesting posibilties alongside kaldorei. It would be actually pretty nice reunification and culmination of kaldorei storyline. On the Horde.... not that much. San'layn on the Horde would be in similar position of two distanced groups of same nation getting together and overoming their differences.
So yes, I would still accept Alliance Nightborne and Horde San'layn.
Good observations. It makes my theory even more possible
Yes, some of us eventually find void elves cool. They have their potential. Speaking for myself, I wouldn't say I'm very satisfied with them, I'd still welcome some more customization options for them, but I generaly like them so far. As you say, lack of lore is definitely one of the biggest issues with the race.