It depends on the Pact, but the soul is the target
The core nature of a Soul Pact is where the benefits to the pact-bound are detailed, the idea that the Demon can claim the soul of the pact-bound is stated, and the document is signed in blood.
In Dark Eras, page 246-247 describes the idea of Pacts before literacy, wherein Pacts' details aren't preserved as part of the signed medium:
Modern Unchained don't use meaningless marks because it makes the mortal more suspicious and the pacts less fungible.
In other words, the idea of finely-worded pacts increases the value of the Pact to the Demon; ergo, the most cautious Demon would ensure they cover their bases.
Standard disclaimer: I am not a lawyer.
For our example, Demon Deb has a Soul Pact with Mortal Marty for his immortal soul.
- b)Devoured/destroyed soul
If there's nothing left of the soul, the Soul Pact's value would have the current value of Enron stock.
- a)Stolen soul
In this case, the ownership still transfers with a touch. It may be that the soul is not the last place it was left, but if Deb can find it, it still belongs to her , now. (It might be worthwhile to include a clause of "the pact-bound cannot, through action or inaction, assist or encourage their soul to be stolen")
- Soul gets swapped.
So, Deb intends to overwrite Marty and step into where he is in life and reality. Deb touches the body she think's Marty soul is in, but no dice; nothing happens. When she tracks him down, rather than a new and powerful muscle car for instance, he's now got a beat up little Smartcar.
If Deb is desperate enough for a working set of wheels, she may have no choice but to cash in. If Deb is angry enough, she may cash in anyway to obliterate Marty in revenge.
Or, if Deb discovers that Marty consented to such (perhaps to spare his family) then she could easily pursue revenge against those he cares about with little options by him to retaliate. Other than, of course, having his soul restored, in which case Deb can still replace him.
On one hand, your average Demon isn't going to know much about the moonborn or how tasty souls ares to Mummies and their ilk. On the other hand, most are fairly paranoid.
On the one hand, cramming Soul Pacts full of "and the signer shall not undertake or assist any endeavor to alter the condition of his soul, else all benefits from this pact are null and void" could mitigate the risks to the pact-bound's soul. On the other hand, Soul Pacts usually target the desperate, and phrases like that might stick out and encourage investigation once desperation wears off.
Ultimately, though, until such a time that a Demon can redeem a Soul Pact, their potential gains are in flux, and deserve at least occasional, cursory attention.
As a personal note, if something interferes with a single Soul Pact, it's probably a coinicidence and worth letting go. However, if another Soul Pact falls to "force majeure", then either disproportionately retaliate or flee.
