and were about to be forced to give up adventuring, because they could barely handle D rank quests, before they heard rumors about her potions.
I do not think they rushed her through evaluation, most people would start at E and rqank up as they completed quests. They just never reported her as taking part in the quests since she "did not contribute" because healing people does not count in the "Hero"'s mind only dealing damage. He basically just treated and thought of her as a tool and slave and not as a person or party member. It seems she even had to do tasks like cook their meals and prepare camp including their sleeping area with them doing nothing outside of combat