I remember that scene, but my interpretation was that the 'spider drawing' as an indicator to establish that Henry was already bad, and sort of 'infected' or under the influence of the Mind Flayer, and it wasn't Brenner or the experiments that turned him bad, as a contrasting element, since that was something Eleven was concerned about.
My assumption was that the Mind Flayer has existed since the creation of the Upside Down, which predates Henry even being born.
Seemed like her mom was abusing her. I got the sense that she was a contestant in those child beauty pageants, and she had bulimia, hence why she was in the bathroom stall, and her 'mom' attacked her, questioning what she was doing in there ... that was just my read on the situation.