But Bonnie wasn't, was she? Note in Skin Game Lasciel referred to her as her child, not Lash's child ...
That Bonnie was there -- but the sigil was gone -- tends to show that Bonnie isn't a viable conduit!
Lasciel had no idea that "Lash" existed. That thing was "Lasciel's Shadow" as far as the Fallen knew (or cared, other than in a tactical sense); so of course anything of Lash's was -- to the Denarian -- Lasciel's...
there was no Lash to them.
... It was unlikely anyone was going to come for it. Maybe Anduriel knew ...
Anduriel certainly knew. Nic/Anduriel
gave the coin to Harry,
watched him take it; Harry and Nic made eye-contact! If Nic or Anduriel ever concluded that Lasciel's Shadow had somehow failed to sway Harry (or that there was a higher priority for the coin) they knew
exactly where to go to get it.
If I were in Harry's shoes, I'd be quaking in them: the HEAD of the Denarians, the sneakiest and most long-range of them, thinks I've got one of his assets stashed away in my home... And the building is full of bystanders/hostages.
So long as Harry believed the Fallen still had some connection to him, he wanted the Coin where he could Take Steps. Stronger containment. HTHP BARS press. Sneaking into a Nasa facility and sending the coin on the next Voyager interplanetary probe, etc...
Once he was confident the influence of the Fallen was gone, his own responsibility was over; then he gave the coin to the Church, to protect the bystanders.
... Maybe Anduriel knew, but then he must have also known that the Sword of Faith was in plain sight in Harry's umbrella stand for years, yet no one tried to steal it to destroy it ...
Actually, I'm pretty sure nobody else knew (in particular, Nic or Anduriel). It's strongly implied that the sword was simply never noticed/spotted by anyone; and Nic isn't likely to have been able to insert many (if any) agents into Harry's place.
Plus, I strongly suspect "stealing a sword from a non-wielding / non-Knight guardian" might just be the sort of "rule-breaking" that lets archangels step in to counterbalance it. Given the sheer power the 30 Denarians possess, and the effectively-eternal lifespan, they'd have been
repeatedly able to attack and steal the Swords, across 2 millenia. That they
haven't suggests there's a good reason not to (at a guess: Divine Providence insures that such a project
Does Not Turn Out Well for them; no matter how careful their plans, it's worse for them to try for a Sword that way, than to wait for a Knight).