One question. Umm... What happens to zombie-san when summer finally comes?

Move to north pole, get job with Santa

i think it might just be that he doesnt feel anything coz he is dead, not hot or cold.so summer wont really matter

The very last panel in the extras gives you a clue. Nice summer sun, zombie-san dressed in winter gear, boyfriend removes winter gear, zombie-san melts, boyfriend wishes he could faint once again.