I don't know about all the coins, but Nick and Tessa for sure.
The Fallen in the coins are significant on Demonreach-level; Nic and Tessa themselves aren't even close. Tessa is a wizard (Council-level, but nowhere near, say, Morgan or Luccio level ability) with some physical enhancements and the ability to turn into a swarm of bugs. Nic has super shadow spying abilities and a really powerful magical item, but that's about it.
Naagloshii are minimum security in Demonreach. One of those would easily, trivially destroy Nic or Tessa - probably both of them working together. (Even Nic, actually, given that naagloshii have intellectus about how to hurt people.)
A more practical idea, it seems to me, and more moral, is Harry's approach. Take the Coins, encase them in blocks of high-density concrete, that sort of thing. Make them physically harder to touch at all.
Yeah. Put them in concrete blocks then drop them in a deep part of the Pacific. That ought to work for a while at least... just watch for people sending submarines into that area...
One would wonder why an uber-powerful entity like Hecate WOULD split her power up?
Maybe she didn't, intentionally.
While it would go against one of my other WAGs ("MW is the original Hecate, thus an Ascended Hecatean Hag, thus not Fae and that's why she can touch iron") - maybe the original Hecate was killed on Halloween in some way that forced the split. (Killed by two people who divided her mantle among them?)
Immortals trade bits of mantles on Halloween, according to Bob. Hmmm... maybe the two WAGs are actually compatible - maybe the original Mother Summer stole half of the original Hecate's power through some kind of ritual or something (without killing her).
The later delegation of power from Mother to Queen, Lady, Knight on each side IMO is a simple matter of getting around the inherently limiting nature of having "reality crushing" level power -- and it's probably not that much loss anyway. If MW is an order of magnitude stronger than Mab and Mab's an order of magnitude stronger than the Winter Lady, MW's only lost 11% of her original power.
What if ... the Blackstaff was the actual Staff portion of the Spear of Longinus?
IIRC there's a WOJ hint along the lines of "Celtic lore around 1065 AD", so probably not.
This could work too, though Id expect that particular shift to center more directly around the Crucifixion, given that all Objects with actual, innate Power (save the blackstaff) all come from that stage.
Medea's bodkin would be significantly older - Medea ought to be from the heroic age of Greek myth, around the time of the Trojan War - say 1300-1200 BC.