that shot under the kotatsu doesn't seem to be physically possible. Gran's not sitting on a legged chair and Izmar, while being a Draph female isn't small enough to sit upright under one.
You can see in both page 7 which is the one I think you're talking about along with page 9 especially that he's not sitting cross legged, there's clearly a cutout under the kotatsu so you can sit on it like you would on a normal legged chair, or sit straight up like Izmir did.
Unfortunately that also means comfortably sleeping under it without suffocating, being upright, or being a living bridge would be impossible so that's actually kind of low tier if it doesn't have a closing trap door or something that also lets heat through if the floor of that cutout is where the heat is coming from.
"you were together and you're flushing sweating all-over the place in the winter? -yeah, that kotatsu was way too strong, it was hot and exciting -... ho, really?"
how stupid do you need to be to not guess immediately?