First off, my apologies if I've either misremembered some elements here or if I've completely missed something which renders my view here irrelevant.

My partner and I just couldn't get over the scene where Harlan "dies". It seemed so rushed from a directorial point of view, shoehorned in where it didn't really make sense or have any lasting emotional impact.

We see Harlan running away without any direct provocation- this is fine, given what's been happening in his household. Vanya follows him, he has something like a ten second head start. We then don't get any indication that she had real difficulty following him, or that any time has passed, and then he's dead at the bottom of the pool. The overall effect on us was just that Harlan was alive, in the house, and in under a minute has managed to kill himself at the bottom of a pond we didn't even know existed. No setup at all (Harlan wants to play there, Sissy or Vanya tell him its dangerous), just Harlan taking a miniscule window of the episode to, as far as we're made visually aware, run directly to somewhere and kill himself.