I think we haven’t seen the last of the castling device. Consider the following:

• Back when castling devices were introduced, Hober Mallow demonstrated that only one party needed a castling device for it to work;
• Foundationers were somehow able to move from the planet’s surface to Invictus after the empire had already blockaded Terminus;
• Bel Riose felt like it was all too easy, he was worried he was being led into a trap;
• There were really a lot of devices in that bin that Poly knocked over in the church;
• Demerzel, after meeting with Harry Seldon in an enviroment where the latter had full control and could easily sneak a message past Day, suddenly felt like she had to be elsewhere.

I think the last episode is going to reveal that it all indeed was a trap, and that a lot of Foundationers have castled aboard Shining Destiny, stranding part of Bel Riose’s crew on Invictus and the planet’s surface right before both are annihilated. Hober Mallow ends up stabbing Day, thus piercing his hide and requiring Demerzel to decant a new clone Cleon.