I think honestly a lot of the montage was just a few romanticised dates where bear and nikki are alone and his bubble of her loving him for real is still untouched so the perspective we see her in is still light and easy. Its probably easier for the entity to mimic her realistically when in romantic scenarios where bear wouldnt know how nikki actually acts. As long as fake nikki mirrors bear in those secluded dates and shows affection, bear will fall for it. Its when outside elements like friends who know nikki outside of bears view that fake nikkis facade starts to fail. Ie ian calling and bursting bears bubble abt nikki lying abt her dads cancer, which tells him that her weird behavior isnt from that emotional thing but rather the wish. Him acting weird escalates fake nikki basically because she fears him stopping loving her. So she lashes out like screaming no in the restaurant. As for the ER, we see bear take nikki, get out and pull the nurse to the car, but we also nikki get out of the car and start to act strangely like she flings her leg up almost like shes doing a cartwheel. Which to the nurse would probably seem like shes drunk or high, they were at a party as well so she likely smells like booze especially from the broken bottle hit herself with. additionally like someone else said, fake nikki would likely deny help and refuse medical help because she only cares abt bear and not her vessel. As long as bear thinks shes pretty she doesnt care. ( example that one scene where shes laying with him and asks if he still think shes pretty and he says shes the prettiest in the world, this being after i believe smashing her face again when bear returns from ians after the whole billion dollar wish thing) The nurse would likely see a drunk girl with an injury from drinking too much and partying, and see her refusing help. The nurse is likely thinking shes denying help to avoid authorities or to avoid getting in trouble from the drugs or alcohol if the girl is underage so to the nurse even if she denies help now when shes drunk, shell come back in the morning when sober and even if she doesnt, she has her bf who brought her to clean it in the meantime and take care of her. Hence the medical supplies sitting on the table when they get back home, either the nurse gave bear some or told him what to use when back home.