@Yautja
I'm with you when you say that a romance between Mashuu and Satoko would feel unnatural or awkward.
Imo, the author doesn't know whether she wants them to either be friends, confidants or lovers. Granted, I'm fine if they don't end up together because I don't see their potential romance as the main driving force of this manga. Their relationship is indeed the focal point of the manga, as for how that relationship develops is what I'm assuming we're all here for.
If I'm being honest, to me, his chemistry with Satoko isn't geared towards romance either. The feeling I get from them is two people who can fill the gaps in each other's hearts and I don't mean in a romantic context. She provided him the attention and motherly love he was desperately seeking and his innocence gave her a feeling of fulfillment/provided for her a reason that gave value to and validated her existence as she was at the time, an empty shell that's just existing, but not really living.