I'm team letter, and also can understand why Verity wouldn't write "exercise practice" on it. I like to dabble with my own dark drabbles when I want to get into the head/mindset of a villain. The thought of marking it as an exercise never crosses my mind because no one ever digs through my belongings, and I trust them not to do so. Verity does establish in her manuscript that Jeremy doesn't read her stories and she already trusts him a lot.

It's also a writer's thing to print out manuscripts so they can look at it through a fresh lense. Another trick you may see from writers is changing their font, but printing is considered the standard. Although, I do want to add... Mine and Verity's practices, while similar, are not the same. I stay very clear from real life people and settings.

With the letter being in the floor boards... I always assumed that was meant to be an endgame move. As in, Verity would take it out once she was ready for it to be found and she wasn't expecting it to end as it had.

I do agree with your other points, though! The book needed a bit more refining, I think, but it's still an enjoyable read for the most part.