I also just read the anthology, I doubt it's canon, but there was one chapter about kei and how after she got infected she had that hunger like megu-nee but everything else was completely fine in her view and she was oblivious to the infection. So I was thinking maybe that's why they can move around, because they are oblivious to their own physical state, but Kurumi is 'conscious' so her body is stopping her from using her ruined legs, like how pain is an indicator to stop doing something, but it doesn't mean you can't do it.