Woah, the foundation of your premise is off, so the rest of the questions you ask are rather moot.
Just a warning, incoming wall of text. But we can break this down into three parts at least.
If the Mission Table missions in BfA are considered canon:
Here's the thing about Blizzard and number losses: Blizzard has never cared. For all the people that died on Teldrassil, the Night Elves will still have enough forces to contribute to the Battle for Lordaeron. And even more importantly, the Azeroth at War: Kalimdor on Fire achievement.
The Night Elves push the Horde back. Clearly not sequestered to Darkshore, the Night Elves rally at Maestra’s Post, Forest Song, and Stardust Spire; take Astranaar back; support Big Baobob at the Shrine of Aessina; assail Warsong Lumber Camp, Splintertree Post, and Silverwing Refuge; and even infiltrated and sabotaged Bildgewater Harbor.
On top of that, the Night Elves still have plenty of holdings and resources that so far have had no mention of the war even reaching them: From as far north as Starfall Village in Winterspring, to all the way south to Feralas, and - with Malfurion squarely standing for the Night Elves now - Moonglade, Mount Hyjal, and even Felwood.
And what’s even more impressive to me out of all of this: Sylvanas had time to prepare for the Battle of Lordaeron. Time to plan to abandon the Undercity and evacuate her forces to Orgrimmar. That means that the Night Elves are not just standing against four Horde races - the Orcs, Trolls, Tauren, and Goblins - they’re standing against five Horde races as the Forsaken are now stationed in Orgrimmar, too. And even against five races of the Horde, the Night Elves hold their ground on Kalimdor.
So despite her successful surprise attack on the Night Elves, despite her shifting her entire bid to breaking the Night Elves by burning down Teldrassil, Sylvanas fails, and the Night Elves not only take the fight to Sylvanas and force her to abandon Lordaeron, the Night Elves stay to fight an keep Kalimdor at the same time.
Malfurion Stormrage said during the War of the Thorns, “A war is not won or lost in a single battle. Our day will come.” Sylvanas tried to win a war in a single battle, and failed.
The Night Elves were still holding strong on Kalimdor.
Now, even if we disregard the Mission Tables, we can look at the Darkshore Warfront and the events of Tides of Vengence and how they compared to the War of the Thorns:
From
A Good War:
When the final numbers were tallied, there would be more slain Horde than kaldorei.
From Elegy:
All who could walk were pressed into service. Even those generally regarded as civilians—tailors, food merchants, innkeepers—had learned over centuries how to fight well enough to defend themselves.
...
Delaryn watched, feeling wretched, as those she had ostensibly been sent here to protect joined their Sentinel sisters in racing silently across the bridges, armed with bows and daggers.
From Terran Gregory at https://www.youtube.com/watch?v=aEHc8le_Qpk&t=10m04s :
Here name is Shendruk. She was created for this cutscene. We imagined that this is someone who’s been actually probably a soldier on the battle lines in this region, you know, between like the orcish line of going into Ashenvale. She’s probably a character who’s been there, right, and that had some experience. And I know there’s been some conversation about the way she talks about Night Elves. Part and parcel, right, when you’re not talking about the game mechanics, right - we know that Night Elves are insanely powerful - but part and parcel, if you’ve lived in this world, there’s going to be a lot of Night Elves in this forest that are just civilians, right? When she’s been patrolling that area, by and large, she’s probably only witnessed, you know, conflicts between the normal Night Elves. You know, the ones that just live [there]. You would have never encountered the Archdruid. You never would have ever encountered [a “single Night Elf” that “could do all that”], right? So a lot of her impressions are just based on like, “Oh, yeah, I’ve seen- I’ve had scuffles with the Night Elves for however many years and I’ve never had any trouble with them,” right?
From Terran Gregory at https://www.youtube.com/watch?v=aEHc8le_Qpk&t=52m58s :
She’s very dismissive. And the faces even, right? Incredibly well animated. While he knows- * Points dramatically at Nathanos * - He knows exactly how serious this is. And she’s all like, “Pff. Whatever,” because again, as we talked at the beginning, she has dealt with, you know, ordinary Night Elves probably her whole career defending, you know, the Barrens fronts. Never personally encountered Malfurion. But he has * Pointing at Nathanos again. * He knows exactly who he’s talking about.
From A Good War:
Sylvanas Windrunner took a deep breath and then hissed it out in frustration. “If we have no other options, I will handle them myself.”
Saurfang said nothing for a while. It was a bad idea, but at the moment, it was the best one they had.
Saurfang and Sylvanas had discussed strategy and tactics for days, and it had become clear that there were two huge, inescapable points of failure in their plan: Malfurion Stormrage and Tyrande Whisperwind. The leaders of the night elves were powerful, dangerous, and perhaps even unbeatable on the field of battle. No matter how surprised the kaldorei would be by this attack, those two would be a terror for the Horde once the fighting began. They had lived for so long, and survived so much, that Saurfang had to consider the possibility that they could hold off the Horde long enough for the Alliance to send help. Ashenvale was their land, after all. They would rally nature itself to their cause.
Sylvanas could match one of them—perhaps—but even she knew that taking them on by herself was . . . not an ideal tactic.
And you know who’s not at the Darkshore Warfront? Sylvanas. And rally nature is exactly what the Night Elves do when the Alliance takes Darkshore back, undoing the damage the Forsaken and Horde did when they held Darkshore.
But to tie everything together, in the War of the Thorns, the Horde lost more people than the Night Elves did, even when among the Night Elves fighting were tailors, food merchants, and innkeepers. And this is reflected in the understanding of the Night Elves held by an orcish lieutenant stationed at Darkshore now in her history of mostly having fought with civilian Night Elves. And that’s not even the same force the Horde will be facing now when the Night Elves return:
From Terran Gregory at https://www.youtube.com/watch?v=aEHc8le_Qpk&t=58m07s :
It’s very much Sentinel and Warden inspired. Right? And Wardens are, as we know, the keepers of vengeance. And while they were crated very specifically they’re still, you know, at their core a Night Elf institution. The concept of wielding vengeance as a weapon. And I felt that actually factored into this. This was just my feeling in putting this story together. If they are capable of being empowered by vengeance, then that means they’re more powerful after they lost Teldrassil than they were during the fight of Teldrassil.
Oddly enough, as far as a timeline of events, I think the Horde’s version of the warfront played out before the Alliance’s version, despite that the Alliance gameplay wise had their version of the warfront available before Horde players got their turn.
We know the Horde faces Night Warrior empowered Night Elves in their introductory questing while being lead by Nathanos, and we know Belmont is captured in the Alliance introductory questing, and freed in the Horde’s version of the warfront.
However, neither Nathanos or Belmont show up in the Alliance’s version of the warfront. And in the Alliance introductory questing we had just secured Bashal’Aran, only for the Horde to be holding it during out warfront and we have to take it back again.
Taking all this into consideration, I believe the timeline for the Darkshore warfront most logically would be:
-
The Alliance introductory quests from “On Whispered Winds” to up to Stage 8: The Night Warrior in “In Darkest Night”
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Malfurion’s assault on the Horde caravan during the flashback in “Terror of Darkshore” before Malfurion meets up with Tyrande and the Alliance player during Stage 9: Nowhere to Run of “In Darkest Night”
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Then the Horde introductory quests from “Trouble in Darkshore” up to “Where Hope Dies”, which would run concurrently with the Alliance questing from Stage 9: Nowhere to Run in “In Darkest Night” to Stage 11: The Queen’s Pawn.
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Then the remaining Alliance questing in “In Darkest Night” to “We Are Coming” matches up with the Horde questing from “The Dead of Night” to “Warfront Preparations”, with the ending of “Terror of Darkshore” taking its place in the timeline here as well.
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The Horde warfront, where the Horde saves Belmont from his capture in Stage 9: Nowhere to Run in “In Darkest Night” and takes Bashal’Aran from the Alliance forces that were there from Stage 7: Zenith in “In Darkest Night”
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The Alliance warfront, taking Darkshore back from the Horde again.
Two additional conclusions I would make is that Nathanos himself might have been run off of Darkshore after the Horde’s warfront, probably by Tyrande and Malfurion, due to his involvement in the Battle of Dazar’alor. And additionally, where people speculated the the Night Elves and Gilneans departing for Darkshore would have weakened the Alliance’s efforts at Dazar’alor, this did not seem to be the case, as both Night Elves and Gilnean forces contributed to the feint in Nazmir that allowed the frontal assault on Dazar’alor to be successful.
Halford Wyrmbane says: My king, the attack on Zuldazar was a resounding success.
Master Mathias Shaw says: The Horde is losing on all fronts. The Alliance should achieve victory in a matter of weeks.
Nathanos Blightcaller says: My queen…reports are coming in from all outposts. The Alliance is tightening its grip. Victory is within their grasp.
Finally, as a conclusion for Darkshore we have this cutscene and quest datamined that haven’t been implemented yet:
TL;DR:
The Night Elves do fight off the Horde during BfA even after the events of the War of the Thorns and the Burning of Teldrassil.
