With so much of the League in custody, you've got plenty of chance to practice the arts of intrigue. You perform dozens of interrogations, then Mindhole the participants and interrogate them again. You turn them against each other, against the League, against their backers; you pull every scrap of information out of their head over and over and over. You engineer a phoney escape attempt, pretending to be in the previous Spymaster's employ. Then you engineer another, pretending to be in your own employ, and it's amazing how many of them would agree to become a mole in the League if offered their freedom - if it wasn't utterly dismantled, it'd be trivial to destroy it. You see their faces fill with desperate hope as they slip out of their cells and down the corridor, and see their expression fall as they run into a wall of Greatswords waiting for them.
After you've convinced three of them in a row that you're their long-lost and long-forgotten sister by using the details of their youth they told you before you Mindholed them, you realize you've reached the limits of what can be learned with a captive audience like this. But you've learned so much about what makes people tick, and what would make them break down.