Squaring Thomas with Winter?
He owed Cat Sith for an offense in Cold Days. With Sith dead or on the outs with the Court, and having been on the job for Mab at the time, then the debt might have passed to her.
And Lara wouldn't want Thomas on Mab's bad side if he was going to be put on trial under her accords.
(Or as others have speculated, his debt to Sith for a guest law offense was claimed to make a guest law offense against Etri. But I'm not a fan of that one.)
I think you have to cast the net a little wider. Think back to Skin Game, Mab used Nic's desire to get the Grail and more likely the Spear by using her Winter Knight to "pay the favor." With Marcone's cooperation, the bank vault was broken into, Harry and company went into Hades and her Winter Knight gained the weapons.. Giving Nic the Grail was a small sacrifice to gain the other weapons, and she knew her Knight could do it with a little help, so did Vadderung for that matter.
This time around, just before hell breaks loose at the embassy, Mab appears with Lara in the Monstermobile.. Mab tells Harry that the Winter Court owes the White Court three favors.. Harry
returns home, finds the place on fire, learns that Thomas is in very deep s--t! Suddenly he is planning an escape for him with Lara..
Then we learn that Thomas has been transferred to the basement where the talks are to be held.
He is under relatively light guard, oh this castle belongs to Marcone.. Last but not least, Mab and Vadderung both seem very aware that a jail break is going on and ignore it.. One wonders what would have gone down, what were they really up to if the Fomor hadn't made such a dramatic entrance?
1] Once again the stage is set on property owned by Marcone, Skin Game, the bank, Peace Talks, the castle.
2] Given the crime he is accused of you'd think there would be just a little more security around the prisoner. There wasn't, it was very light... In fact, stranger still that Thomas was being kept in Harry's old lab, that though stripped, still had an intact summoning circle.. It might not mean anything, but then again it might be significant that that little detail was mentioned.
3] Skin Game, Mab used a favor owed to Nic to gain access to weapons she knew her Knight wouldn't pass up. In Peace Talks, Mab once again is pulling Harry in to pay Lara of the White Court three favors the Winter Court owes.. Then all hell breaks loose.. Oh and in both Skin Game and Peace Talks Vadderung is also in on it... The guards in the castle guarding Thomas, are his.