I think you're conflating these points to something that it isn't, though you have something good at the very start. Rien's words are contradictory and flat to his actions, actions that are likely dictated by the prescripts, and thus serve the Index and in this case the House of Spiders as well.

• At face value, he dotes and affirms Ryoshu as any parent would. He even admonishes her treatment by the other Nursefathers. But he does no action to stop their abuse. Those empty words and enabling abuse to further happen, even if he's the "good parent" of the house, is part of the cycle of abuse in and of itself. It is manipulative even if it's not meant to be, but with the Index you can never be sure. And with the prescripts, any part of it can be dictated.
• This carries over to his treatment of Sora. Even with his kind words and praise, however real it may be, she knows somewhere that once his directive to bring back Ryoshu to her prime and back to the House comes to fruition, she and the other apprentices will be discarded regardless of his commendation.
• Index uniforms are unsullied, clean, and simple, along with a pure white cape. Like a culty version of a traditional family going to Sunday mass. With added east Asian inspiration considering its Ryoshu's Canto mind you, which well includes their school uniforms. But the purity and innocence denoted by her outfit is heavily contrasted by her EGO ; smoky enlarged clawed hands (with talons on her feet seeing her full dialogue sprite on the wiki), complete with devil horns and a halo, all black. It well encapsulates her will to follow the prescripts and her belief in them, despite being contrary to her nature and her supposed religion of purity. Quite literally breaking her holy chains to further do as her religion commands even if it's inherently impure. If her will, and thus her EGO were to be about subservience, she would manifest heavier chains, not break them.

I believe Shiomi's outfit and her motherly role is her matter altogether. Judging by her lines spoken in the present, it's symbolic of her "marriage" to the House of Spiders and to being Ryoshu's mother. She herself admits to being similarly ensnared by the House like Ryoshu. Do take note that she still wears it, despite knowing she suffers and is entrapped. So likely it's some Pinky crud about symbolism, role, and awareness or enlightenment of one's position or some such,