Is it permissible to wear a hair scrunchie that does not show its bulk from behind the shayla, or one that shows its bulk only in front of women and unmarriageable kin (mahrams)? And in this case, does the saying of the Prophet, peace and blessings be upon him, "Their heads are like the humps of tilted, leaning camels," apply to it?

There is no harm in a woman using a hair clip to gather her hair, because it is merely a tool placed at the nape of the neck, and it does not fall under the prohibition mentioned in the hadith (definition) "their heads are like the humps of camels," which refers to making the heads appear large with head coverings, turbans, or hair extensions that are wrapped around the head and give the illusion of abundance. The simile of the humps of camels specifically refers to what is at the top of the head and gives the illusion of abundant hair, and a hair clip does not fall within this meaning.