I will say re: the lightsaber, TLJ implies that Rey will rebuild it along with the Jedi and the rebellion. Her thing is taking the past and moving forward with it and making it her own. So rebuilding it serves the story of the character, though if the new design is basically just the old lightsaber again with minimal difference it’d be a less effective way of making that point than a more significant change to the saber (but you don’t want to go too far and make it look completely different).

Kylo is about killing the past. Which is why the mask being back is a headscracher. There’s probably a few ways you could logically explain it (good intimidation that was working before he broke it), but it’d muddle the symbol it holds for his character. Then again, we don’t know for sure where his character is at in IX yet.