I think that she resigned from her position right after that fight with Zaraki. She already found a worthy successor, all she needed to do was to prepare herself and await his return... unfortunately, when he returned he still was limiting himself and seemingly she was forbidden from directly confronting him (up to now at least). In a way, her dream was already half complete by finding him and seeing as no one else strong enough was left to bring her joy (Yamamoto changed after the war and all other strong people went to Zero Squad, or simply were weaker than her) she devoted herself to healing arts (in order to prolong their future fight). Seeing as both, Kenpachi and Unohana, have scars, means that she didn't know healing arts at that time, or at least wasn't at the level at which she is today (to be able to quickly heal wounds without leaving scars). It seems that the Zero Healer taught her basics and left her some techniques for her to master in time. Perhaps the purpose of the 4th squad wasn't to be healers, but to decipher and obtain those techniques for herself (assuming that she wasn't able to do it at first). Of course experience in healing others certainly polished her skills.
Originally Posted by paulbee