Journey Before Pancakes

Most of the book was pretty clearly her downward spiral. That said, I think you could also argue that she did show signs of improvement, or at least signs that she was breaking the trend. There were a few points that show this. The one I'm thinking of was when she had to fight back all the other selves that were trying to pop up near her feet. She fought them back and pushed back the urge to split into an even worse mental state. So the way I see it Oathbringer was supposed to show some of the worst parts of er downward spiral, and shows her pull out of it a bit. It still doesn't leave her in a great spot, but she stopped herself from losing it entirely, which was the path OB seemed to be taking at first.

I doubt Navani would ever become radiant. She might be damaged enough that it's theoretically possible; but I don't think she will. Mostly because her primary interest includes trapping spren, which would likely make honorspren uncomfortable. The only exception I can think of would be if for some reason trapping (lesser) spren is seen as a virtue by some other spren.

What's to say he wasn't just omitting information? That seems like a Hoid thing to do. He basically spends all his time selectively giving out information.

If you're curious, that's not just specific to aluminum. You can do that with steel as well, as one example. It's called work hardening.