I love this concept and art direction!
As far as gathering clues goes, I think it would be more effective to flip the shadow's replies from "I don't understand what you mean" or "You're not making sense" (odd if I'm giving them back their own words) into something like "You only repeat words without understanding their meaning" and "That's not what this is about". I think it would make the shadow's story feel more like a puzzle the player needs to interact with and decipher to earn another glimpse and earn their next piece of the puzzle in the family's story. The way it is now they seem more confused than cryptic.
Also I think organic hints would be useful (though this is harder to implement) - If I've failed to highlight a key word in a previous passage and continue to focus on useless words, could they reply to a useless word back with that not-fully-utilized passage? Maybe not every time, that would make it too easy, but after two or three misses in a row? Like "It's not about [useless word], you don't follow. [repeat previously unlocked passage with unhighlighted key words]"
I can't wait to see where this concept goes!