Accidentally introduced Szass Tam into Tyranny of Dragons....

TL;DR Cult of Dragon / Wizards of Thay have captured my level 19 party. Meant to have cult leader be the mastermind (which would have made plenty of sense) but "accidentally" had the leader of all of Thay be there instead. Looking for good story-hook to have him involved to keep from having to retcon half a session (and possible compelling storyline)

First off...... I sincerely and genuinely apologize for being wordy in the following....

Background for context: My 5 players are all level 19 (leveling to 20 next time the can take a break). They've gone through all of the official Rise of Tiamat material and have finished several months of homebrew stuff to get them from lvl 15 to 20, culminating with them gathering spell components for Blackstaff to protect Waterdeep. Some months back, the wizard had gotten disintegrated by the cult and the cult of the dragon had recovered his gear, notably including a Staff of Power, +2 Ring of Protection, Ring of Spell Storing, a bag of holding, Ring of Spell Turning, and an Archivist's key. In the months since that had happened, he had found replacements or substitutions for most of it and more than one legendary item that would more than make up for it, but his character (very good RPer) really wanted to recover the archivist's key for RP reasons.... so he used Wish to get a simulacrum and the sim's Wish to pull out a veeeery carefully worded and well thought out 120-word spell to recover all his lost gear without any ramifications that he would consider negative. The player had obviously put a lot of thought into the wording, and he pulled this out in the middle of our session, and while very quick on my feat, I couldn't immediately figure out how to do something unpredictable without holding up the session while I put thought into it, so I basically teleported all his stuff to him like he asked, along with all the cp, sp, gp, & pp that was in a pile with it, having been hoarded for Tiamat's return (the player was aware of it's location and surrounding already). It goes without saying that the party was aware they were immediately being scryed on, and they immediately went on a several-hundred-thousand-gp shopping spree, not having considered that, perhaps, the cult would want their ill-gotten gains back (and the dragons that had donated substantial portions of their hoard).

Fast forward an in-game week and they had just completed a difficult quest and were teleport-circling back to their tavern (modified Trollskull in Waterdeep) when they found themselves, instead, in a 10' wide cell and a Rube Goldberg machine then exposed something akin to an artificially created beholder's eye to nullify magic in the entire cell. My intent was to have Severin Silrajin be the one to have orchestrated this (after all, capture, neutralize, and keep alive is safer than killing just to be resurrected somewhere else and come back) with the help of the Thayan wizards allied with the cult.... however it was late at night and I was tired when putting tokens on the board and my tired brain put Szass Tam there, instead, having cooked up a character sheet appropriate for him and everything.

It was only during the session, when he introduced himself and the appropriate amount of "oh crap" came from the wizard, I realized that.... lore wise, there's no official "he's doing this" during the time period for the Tyranny of Dragons (Hoard of the Dragon Queen + Rise of Tiamat) campaign time frame. And not only that, it's specifically mentioned that it's a "splinter sect" of the Thayan wizards that joined the cult. I spun it into a "You have been a thorn in the side of some of my brethren for quite some time. They approached me and we came to an accommodation, so I have captured and neutralized you and as long as you don't do something stupid, I won't kill you" kind of deal. My original intent was for the party to realize the Ranger/Druid could still wildshape and free the party, they could fight a hand full of undead and possibly discover a secret or two while leaving the tower (can't teleport while inside the tower), and once outside, they find that somehow the cult/wizards had teleported their entire tavern and looted it for anything worth value (and had done something the Wizard would count as a "too high" price).

My question is this: I really don't want to have to ret-con something like this if I can help it... Can anyone think of any way of spinning this outside of either A) Szass will release them if they do a job for him or B) Szass is really on the side of the cult?

My off-the-cuff thoughts are that if the official module has the players standing a decent chance (though not guaranteed) of defeating Tiamat at level 15... and my hella-beefed-up CR 30 Mythic Tiamat should be defeatable by the level 20 extremely well equipped party (with some npc help at times).... then demigod status or not, Szass believes that Tiamat is defeatable and he may plan to steal the cult's gold to fund Thay for a few centuries or something.... idk, I'm spit balling, but would love to hear what anyone else thinks!

(Also, here's my hand-crafted Thayan Tower map (with Forgotten Adventures assets) if anyone wants it. Go wild. Will include the DungeonDraft file as well for anyone who wants to edit.)