That'd be a good fallback, actually, because the devils have a secret reason to want to do that- according to 3.5's Fiendish Codex 2, every deal is building a secondary case for their mortal souls.
It is also possible for a defendant to win her case on merit, only to suffer condemnation to the Nine Hells on unrelated grounds if her corruption score or obeisance score (see page 30) equals or exceeds 9. Much diabolical laughter then ensues.
Currently I think they're fine- the corrupt act would have been allowing the devils to run amok. They... didn't play along
So maybe a devil offers them a really good deal to get them out of this. But what corrupt action could a devil convince the party to actually perform? "Stealing from the needy", unlikely and low-value. "Torture", "Murder", no way...
How about "Perverting justice for personal gain"
I've gotten distracted but basically- if a "third party" devil can convince the party to
cheat at trial, that's secretly corrupting them.
Two ways I could see that going:
An actual rival in the devil hierarchy gives the party blackmail material... or maybe just lies for their benefit (if they don't call out the lies, they're complicit). This is tricky because devils are lawful creatures, but they're also constantly scheming against each other so I'm not sure what they're physically capable of.
A "rival" does the above, and it seems to work, but it's actually a collaborator working with The Alchemist to play the long game on their souls. The case is intentionally lost, but legally, and for evil purposes.
It's not (RAW) nearly enough corruption to actual damn them, but maybe it's still something the devils would do and it gives the party a temporary reprieve if they were otherwise losing the case. If they figure out that they're corrupted, the atonement spell can help but maybe they need to demonstrate they regret breaking the law. since this is infernal law, that might be very difficult