Hm, so this might even be an allegory for dementia and Alzheimer's instead of the usual discrimination interpretation of the zombie genre (at least that is supposedly what Night of the Living Dead was about).
The unstoppable and gradual deterioration of the mind is the most painful, from what I've experienced. That coupled with the odd lucid moments of the person in which they have the presence of mind to understand what is happening and the feeling of helplessness on the relatives' side.
Because the manga gives materialist explanations of what is happening, I don't think there is any hope of the process stopping by itself or reversing it. It should be even astounding Makoto is still doing somewhat well.