As someone who made a similar comment, you summed it up well. The thing that's interesting to me about this manga is how Ayano really feels like a kid looking for any sort of adult. She's not "mature for her age," honestly. That giant red flag in the description is kinda wrong. She feels more mature, but that's cause she's been acting her whole dang life and has had to handle everything in the house on her because she has no present parental figures.
But, like... she writes little notes like "how old are you" or "how tall are you" on little pieces of paper and sneaks it into things Fumi has. She gets excited about pictures and immediately frames them and puts them in important places. She pulls his hair to get his attention, gets excited about matching aprons, doodles things... she's a kid. Like any and all of these moments could be put in one of the 'cute daughter' manga and everyone'd be going 'aww how cute,' cause it's clear she's just lonely. She feels like a little sister, or a daughter, or a younger cousin. And if that was the established relationship, nothing would be off at all.
But it's her manager so all of a sudden there's all these questions about what the heck should be happening here. What it means. What it could be, what it should turn into, what any of this all is to everyone involved.