The Dragonhold prologue is all about getting the Horn of Ja'darri, and it's supposed to be this super powerful tool that will allow us to have a fighting chance against dragons because it can keep them grounded. We saw how essential this was when we used the dragonhorn against Mulaamnir at the end of the Elsweyr questline. But yeah, the only time it's pulled out in Dragonhold, it's just a dramatic device in the reveal that the horn belonged to Nahfahlaar. Sai doesn't even bother using it in the fight against Laatvulon at the end of Dragonhold, which makes no sense. It's almost like once we got the mask, the horn didn't matter anymore.

I did hear Sai blowing the horn in the Kaalgrontiid fight at the end of the epilogue, but he only did it in conjunction with my directing Nahfahlaar to use his draconic power. I kept hearing Tharn yelling at me to either ground Kaal or wait him out, but I wasn't able to interact with Sai to tell him to use the horn. We got through the fight, but again the horn seemed inconsequential and it was all about the mask.

Why does Nahfahlaar try to talk us out of going to Pridehome to counter the New Moon attack there? It seems like he either totally forgets about the mask until we tell him that that's what the cult wanted, and/or he disregards the importance of the mask in defeating Laatvulon because he doesn't think Laatvulon's priest will be able to use it. But his history with Ja'darri should be an indication of how important the mask would be in countering Laatvulon, even if he doesn't yet have faith that we would prove worthy of using it, and it would be a pretty big brain fart to just forget the mask exists or that the Pride of Alkosh has a connection to it and its secrets.

He plays the whole thing off like it wouldn't be a big deal to let some priests die, but that only makes sense if there's nothing to be gained by talking to the priests or trying to save them, or if he has a better idea about how we could more effectively use our time. It could be that he knows the New Moon can't get the mask at Pridehome, which is true, but why then doesn't he tell us to go get the mask from its true resting place? And why, when we do finally talk to him about the mask, does he respond by flying off to throw himself at Laatvulon, when he previously knew that he wasn't strong enough to face him and actively avoided drawing his attention? Has he just written off all of our efforts at that point, since we're wasting our time chasing after a mask that can't possibly help us against the big bad?

Why did the New Moon go to Pridehome after the mask, when Ra'khajin knew from his training where the mask was located and what he'd have to do to get it? It would make sense if Laatvulon just wanted to wipe out the Pride of Alkosh because he feared them, but it's stated explicitly that the New Moon went there after the mask.

It also felt pretty artificial to be stressing about portals in the epilogue when I spent the entire time thinking "okay, so where's Cadwell?" I mean I guess he had to see a man about a horse, but you'd think Khamira might have kept him on the horn (oh hey that's like a pun) just in case the team needed him.

Okay, so I definitely wasn't taking dragon ego into account. I can see how pride and maintaining the appearance of superiority could lead to some otherwise irrational decision-making in the "hey don't look over there, I totally didn't make a really dumb ego-fueled mistake eons ago" vein. And not wanting to rely too much on the puny mortals could lead to the hubris of "welp guess I'll go take on my arch-nemesis myself and see how it goes." And then maybe having it hit home that hey dude maybe you really should form a partnership with this mortal so that you can accomplish your goal together in a way that you couldn't do separately. So I can reconcile that, knowing what we know by the end of it.

I guess mask > horn, but it still feels like a letdown that the horn was so hyped at the beginning and then ended up not mattering at all.

The portal thing is writing convenience more than anything, since like you said it'd be too easy to be able to portal around everywhere. And obviously they wanted to get Khamira's crisis of confidence in there. It just feels contrived that the portal issues weren't about interference from green space rocks, or dragon magick shenanigans, or even alcohol, but the fact that the characters didn't think it was important to make sure their portal specialist was on the scene to begin with. (Hell, I was even thinking at the time we ought to get Gabrielle in there for backup. )