Nimi-san has gone against the Shinsengumi. To avoid anymore problems they want to get rid of Serizawa.
to hang onto their sanity by a thread.
أن يتمسّكوا بسلامةِ عقلِهم بفارق شعرة
di mantenere per un soffio la sanità mentale.
немного сохранить рассудок.