Question More then the spell is intended to do?

Do Sidhe bodies end up being self cleaning when the individual dies because of that biological self destruct spell?

Well, that would explain the absence of Sidhe remains in the fossil record

Or maybe rats and bacteria just find them deliciously yummy and irresistable...

I have one sci fi book in my collection that goes on to explain the reemergence of the Fae and that no fossils exist because they dissolve 100% like leaves one description goes

And I I remember it right Magic force in general is what kept them alive and is a 'binder' so without it they are dust just like humans minus water are just a few pounds of chemicals

Actually, that's not a good example because there are plenty of fossil leaves around -- if conditions are right, they leave imprints on minerals, for instance. It's not enough that Sidhe decompose completely, they have to decompose fast enough, 100% of the time, that there's no chance of an imprint being left behind.

The only example of a Sidhe seeming to die in the physical world so far (Kalrys in "Small Mercies") did in fact dissolve into a cloud of dead leaves, but my impression was that this was due to her quest ending and her spirit being released from its duty. I am pretty sure Kalrys was dead long before meeting Matthew/Monica.

And given some of that Sin has said, I'm not sure if Kalrys was actually Sidhe or not - that may have been Matt's assumption rather than a fact.