Sorry if I sound blunt, but...Have you actually compared the Saints and the Spirit pages? It's pretty clear that all Spirits are blitzed and oneshotted by a Saint-level fighter, much less Saint+ people like Brunhild with Gungnir, Carissa, Acqua of the Back, Knight Leader with Hrunting, Elzard and the Maiden of Versailles. It matters little if they have an interesting ability when they are much weaker, slower and in general lack experience and ruthlessness as fighters. All Saints should be able to kill Kotori, her Regenerationn is not that great if decapitation still works.
It says in Nia's page that her ability can't affect Shido when he uses the powers of the other Spirits, so as an ability that fails to affect people much stronger than yu, how would it begin to affect characters that are even stronger than Shido? As mentioned, Natsumi and Kurumi are killed before they can do anything. Plus, Kurumi's abilities are incredibly telegraphed, and her bullets need to hit the opponent to apply their effect, which is not going to happen with Saints.
Your comment about Spirits stopping their aging means little in this context, Type 1 Immortality is only useful in combat when someone tries to age you to death. It does nothing to stop Saints from stomping the Spirit. And no, there are multiple ways to retain your youth in Toaru, and they don't have to do anything with being stronger than a Saint. Magic Gods. Just because you have Type 1 Immortality doesn't mean you are stronger than Saints and/or close to Magic God leves, that doesn't make any sense whatsoever.
Spritis being named after angels doesn't mean anything either, they are totally unconnected with the concept of God's Right Seat or Toaru's hierarchy of power, so I don't know why you keep repeating this point. As I already explained to you, so far the strongest Spirits are below Saints in power. Some of them have interesting abilities, but still nothing gamebreaking for Toaru.
As for Rinne and Phantom, going from what the wiki tells me about them, I highly doubt that they are anywhere close to that level of power, as nothing they have done so far approaches guys like Fiamma, much less Othinus, who is the weakest of Toaru's Magic Gods.