Dalen will hang back and cast silent image next round to create an illusionary Trinia for the guards to chase after. Once he's sent them in the wrong direction, he'll make his own getaway. Presuming the others all manage to get out of sight, I don't think there's any actual dice-rolling necessary here (Dalen is invisible for the next 3 minutes, and illusions don't allow a save unless you actually interact with them) so unless something unexpected happens Inspectre should be able to resolve Dalen's actions in the next GM update and conclude the encounter.