Nope.
The reality is those coins are meant to be in circulation. Not even Hades can keep them out of play.
This, of course! You're spot-on, here.
Ultimately, the story goes however Jim says it goes. He could leave the coin in the vault; he could have it already out and circulating; he could leave it there, and pull it out when he wants it back. Maybe he has HARRY summon the Coin!
Similarly, Lasciel could be out and circulating (on the "meant to be in circulation" grounds), but we may never see her again if Jim decides a "Harry/Lasciel rematch" would be too much travelling the same ground.
The point is that Lasciel's part in the story has ended. Unless and until Jim decides to put her back into the story, the Coin is in the Vault. She can come out of the Vault if Jim wants her to—she doesn't even have to show up. Just a throwaway line from Nic or whoever saying something about removing the Coin from the Vault is enough to remove her from there. But unless and until that happens, that's where the Coin remains.
Just because the Coins are meant to be in circulation doesn't mean that they can't be removed from circulation—at least for so long that it's essentially the same thing. Enduring until the heat death of the universe or the apocalypse is kinda indistinguishable from "forever" for practical purposes, even if those events happen within a few decades.
The Coins' ability to stay "free" mostly seems to rely upon variables that can be manipulated—almost like an entropy curse. But there have to be things that can be manipulated for it to happen—breaking an aquarium's glass wall, for instance, and a gentle shift along the Coin's axis can send it rolling toward an unsuspecting mortal, like we saw. Held within the Church's version of Area 51, you've got various people you can manipulate (apparently responsible for Nicodemus recovering about 50% of lost Coins), along with environmental effects, electrical circuits to disrupt, fires to start, weather, etc. Plenty to work with.
In Hades's Vault, there's rock, treasure, shades, and a god so badass he wears a crown made out of Deathstone. Yes, mortals can reach the Vault with enough badassery of their own, but we all saw the kind of team it took to get there.
The only way the Coins (remember the Genoskwa had one too, and I don't know if Nic thought to grab his on his way out) had anything to work with at all was at the moment of their hosts' deaths. If Hannah's Coin squirted out through a gap in the molten rocks from the pressure and landed on a pile of other coins, sure, Nic could've grabbed it when he skedaddled. I find it more likely that he grabbed Genny's after Harry got him pancaked—that's something Anduriel's shadow can do, being two-dimensional and all.
But the point is that, unless Nic grabbed Lasciel's Coin somehow, doubling back to the Vault after escaping Michael and Harry, it just doesn't have anything around it to work with that would get it into mortal hands. It's no longer on the mortal plane at all. It'll stay there until another crew decides to pull a heist.
More importantly, it'll stay there until Jim needs it to be elsewhere. It's just too much of a stretch for me to really think otherwise unless it's stated in the text. Speculation is dope, and I encourage it; there are plenty of fun ways to imagine the Coin is out there wreaking further havoc still. But there's just too much against the idea for it to be really feasible to me without confirmation.
Lasciel will show up in the stories if Jim wants to have her there... my guess is that he does, and so she will.
I don't know what Lasciel would add to the main case files. Her arc feels rather complete. We saw her shadow's arc play out between Death Masks and White Night. We saw the aftereffects for several books beyond that. Then we saw what it looks like when Lasciel's temptations work, from an outsider's perspective. And we saw Harry dish out karmic justice (pretty directly, too; he used Hannasciel's magic against her). That felt pretty final to me. Having her return to the main series would just feel kinda like retreading the same ground. Considering that Jim wants to start tightening up future novels, I'd say that yet another round with Lasciel would constitute unnecessary bloat, the type of which you'd want to trim.
The BAT is a different story, though (heh, possibly literally). I have zero doubt that the Fallen will play a big role in the Apocalypse. Lasciel could have a role there.
But where we are in the series, I'm not even sure the Denarians are going to return before the BAT. If they do, it'll be one more round, probably around book 20 (could be the very last one, which might be as late as book 22). It depends on how the next book shakes out, in my opinion; I fully anticipate Peace Talks will be setting the stage for the remaining players for the series and positioning everyone for the last minute scramble before the music stops. Nicky and His Superpals will probably show up before the case files ends, but it's possible they'll be offstage until the apocalypses kick off. (In which case, my WAG is that the order of the BAT is 1. Hell's Bells, 2. Stars and Stones, 3. Empty Night; Hell's Bells will start with the apocalypse of Revelations (or the Dresdenverse equivalent) and the Denarians' role will be center stage.)
