hello! long time watcher of black butler first time reader and i just caught up. something i don't get during the campania shenanigans and i was hoping you would explain. why did undertaker keep going with the first bizarre doll experiment when he found out that ciel and the midford family were on board. If he's cedric or just if he cares, shouldn't he have aborted the experiments and protected them? thank you a lot for answering this and sorry if you said this before but i didn't find it.
Hey Anon! Even if you're not necessarily new to the fandom, welcome to the readers' side, hahaha! As for your question, I'm pretty sure I did answer this here and there once or twice, but I'll sum it up for you here. :)
First of all, while UT's a complex character, unlike some other fans, I never found that anything he does is senseless. Not ever since ch105. However, writing-wise, Yana-sensei can only slowly unveil more and more hints for UT's character and his agenda throughout the arcs.
For example, she can only reveal that UT "can't bear to lose another Phantomhive" after showing us that he cried about Vincent's death in the GW arc and after revealing he's a deserting Shinigami who's trying to bring back the dead in the Campania and Weston arcs. All these hints are revealed one after the other to make the readers slowly reflect about UT's goals and his true identity, as well as his past actions in previous arcs, in order to get to the bigger picture, just like you're doing right now.
As of right now, with all the hints we've gotten, we know that :
UT cares about salvaging what's left of the P family, as much as he cares about bringing back what he can of the ones who are dead (real!Ciel, maybe Claudia, but not Vincent because his body burnt too badly)
UT considers Frances and most likely her kids to be amongst the people he "can't bear to lose"
That much you can be rather sure of and that's the beginning of your answer: during the Campania arc, while our!Ciel and the Midfords were indeed in danger, UT also had to conduct this experiment in order to perfect his bizarre dolls, namely for real!Ciel's sake and for his sponsors, so he made a gamble.
For our!Ciel, he knew that Seb is a demon contracted with him, so he figured that Seb would definitely protect Ciel from the BDs.
Additionally, by checking Seb's cinematic record during the arc, he also found out about the terms of contract between him and our!Ciel (what we read about in ch138). So, post Campania and as confirmed in Weston, UT now knows that Seb is bound to obey and act as our!Ciel's shield as long as Ciel's revenge isn't achieved. That's why UT uses Seb to make sure of our!Ciel's safety.
For the Midfords, it's a similar gamble: ch151 revealed that UT has known Frances since basically her birth, so it seems logical that he's aware she's a very talented fighter, just like he probably knows that Lizzie is just as skilled (could be why she even was recruited by Bravat to protect real!Ciel if UT wasn't always around during the blue sect arc, me think).
Additionally, since UT knows that our!Ciel is quite selfless, I'm sure he also figured out that our!Ciel would order Seb to help his family if necessary:
Lastly, UT found the time to put BD!real!Ciel to safety despite the chaos on board and, since he was looking for a way to sink the ship after the experiment until they hit the iceberg, I'm quite sure he had planned to make sure our!Ciel and the rest of his family would end up safe no matter what.
I know some fans also used to think UT doesn't make a difference between being alive or dead thus used to say that, if Ciel and the Midfords had died during the Campania, UT would have simply collected their corpse and turned them into BDs. Personally I don't buy it, since his bizarre dolls still can't function without regular blood transfusions as of right now, so I'm sure he'd rather have them alive than dead.
Other fans also used to say, long ago, that Frances was in cahoots with UT during the Campania, which is why he didn't have to rescue them, because she knew enough of his plan to manage to rescue her family out of it, but 1) then why did she go on board at all? 2) I think ch151 definitely proved that she never was involved with real!Ciel's revival.
TL;DR I think UT bet on what he knew of at least our!Ciel, Seb and Frances to keep on conducting the Campania experiment while they managed to escape but, since we know he definitely doesn't want to lose them, I'm sure he had a backup plan that involved him rescuing them if necessary.
I hope this answers your question! Have a great day!