. This means that diplomatic PCs risk becoming accessories to their crimes.
Not really,
It means diplomatic PCs have serious leverage.
The villains are such that they don't quite know how human justice works - other than the fact that humans are perfectly willing to come in an murder them at the drop of a hat (since this has already happened).
Industrious PCs can convince this group that giving up the books to Candle Keep and possibly paying reparations (this one would be a tough sell, but maybe there's a PC with serious persuasion in the party), is the only way they get to continue operating at all.
And as already stated, diplomatic PCs are not accessories - there is no interest (more than likely) in a cover up, just recovery of goods.