If you've already hit him in the head, and you don't want to use a gun, why bother using undetectable poison? Just hit him in the head again. It's not like the new drug can cover up head wounds as well, so I'm pretty sure that the whole murder part of the mysterious death will be still self-evident.
That shitty man at the end said that she was "sulking"?! She's literally bound and gagged, crying on a bed, just after a man tried to rape her!! She was powerless! The only reason the rapist stopped is because he chose to, she was completely at his whim. You think that's not going to fucking...
I ended up dropping this. I did like it at first, but the storyline just seem to fade away and it became about him meeting a woman, she falls in love with him, and that was it. Over and over. It was like reading nothing but filler chapters.