1. Ember's POV can't be trusted because she's not from Prythian and doesn't understand the Prythian Fae's facial expressions.

I don't foresee SJM taking the time to write a BC from a very minor side character's POV, which focuses exclusively on Nesta and her emotional state, as well as Nessian, just to shrug it off as, "Oh, Ember didn't know what she was talking about." Feyre was described as having dead eyes in ACOMAF when she first left Tamlin and went to the Night Court. The fact that Nesta is described as having a nearly vacant and carefully blank face is a parallel of sorts. SJM has never used such negative emotion to describe a character before. She went completely overboard in the BC for a reason, so no, I don't think that the takeaway is that Ember's POV is sus. I think we're supposed to pick up on Nesta's negative emotions because it will fuel her story arc next book.

2. Cassian warned Rhysand about his tone when berating Nesta, and he stood in front of Nesta when Rhys threatened her.

Cassian never warned Rhysand about his tone. He very meekly said "Rhys" and then later said something like, "It's been a long night. Let's discuss in the morning." That's it. Rhysand snarled at Cassian and he quickly shut up. That's the most we got of Cassian defending or supporting his mate. Cassian did not stand in front of Nesta during the Rhys/Nesta staredown. He stepped an inch closer to Nesta and then was conflicted about who to side with.

3. Nesta acted impulsively in giving Bryce the Mask and didn't consider that Bryce could die or that the Asteri could invade Prythian until the Ember BC.

Nesta did not act impulsively in giving Bryce the Mask. She mentioned multiple times throughout the crossover the risks involved in giving Bryce the Mask, but ultimately decided to do it because it was the right thing to do. Nesta told Bryce before she even gave her the Mask that she could very well die. They had a whole discussion about that possibility when they were talking about leaving Ember and Randall with Nesta as collateral. So, no, it did not just suddenly occur to Nesta during Rhysand's tongue lashing that Bryce could die. Once again, Nesta mentioned multiple times before giving Bryce the Mask that the Asteri could possibly invade Prythian, so this was not something that just occurred to her during the BC. We literally see Nesta talk through different scenarios with Bryce before giving her the Mask, so no, that decision was not impulsive. We also know that Nesta has a connection with the Mother and is guided by Fate, so who's to say that Larger Forces didn't guide her in her decision? Either way, the Asteri are defeated, Midgard is free, the Asteri are no longer a threat to Prythian and Cassian and the IC treated Nesta like garbage.

4. We don't know why Nesta struggled to remove the Mask in HOFAS.

If Nesta were pregnant, I doubt they would have let her go in the tunnels with Bryce. That would be putting her baby far more at risk than her giving Bryce the Mask.

I don't think Nyx being the one person to make Nesta take off the Mask is nearly as important as the fact that Cassian was not the one who had Nesta wanting to take the Mask off. Nesta had no problems taking off the Mask in ACOSF - she wore the entire Dread Trove with no problem - so her struggling here is just a testament to how lost she's feeling and the fact that she doesn't want to remove the Mask because she wants to disassociate from her feelings.

Bryce and Ember both commented on Nesta's "longing" looks, which is also a sign that she's not in a good place with Cassian. Plus, Ember highlighted Nesta's nearly vacant face, carefully blank face, thin and brittle voice, the bruised/pained/wounded look in Nesta's eyes, the steel wall she had to raise in her mind to hold back a giant wave of raw emotion. Nesta's getting ready for a breakdown, not a baby.

In HOFAS itself, we have Bryce commenting on Nesta's "longing" looks and we have Nesta struggling/not wanting to take the Mask off when she had no problems whatsoever with taking it off in ACOSF. It wasn't even Cassian's name that brought her back in HOFAS, although it was him, exclusively, who brought her back in multiple instances in ACOSF. We also have Ember's "You'll find your way" line that further indicates how lost Nesta is.

Then there's the Ember BC, which highlighted Nesta's negative emotional well-being as well as problems amongst Nessian. Nesta's nearly vacant face, her carefully blank face, her thin and brittle voice, the wounded/bruised/pained look in her eyes, the fact that she had to raise that wall of steel in her mind to hold back a giant wave of raw emotion. Her surprise, gratitude, and "longing" look when Ember defended her. When it comes to Cassian, it's the fact that he let his family violently enter his home, berate his mate, and threaten his mate within the confines of her own home, and he does nothing to support her, stick up for her, or protect her.

I don't foresee any author worth their salt going through the effort to make the Mask essential to Bryce/Midgard/CC only to villianze the one who made it possible to have the Mask in Midgard. Even if SJM hates Nesta, what was the point of having her give Bryce the Mask - just to get off on her subsequent suffering? I highly doubt SJM is going to get into the mind of Ember - a very minor side character throughout three CC books, who has never had a POV before - just to find amusement in Nesta's suffering. That sounds like a bit of stretch.