(This got long 
I’m surprised there was no prior discussion about the actual mystery though, considering a lot of stuff that seems like it at least could be relevant happened in the chapter)
I kind of wonder why they dismissed the possibility of Magata Shiki’s own involvement in her death so easily. While I’m guessing it won’t end up being the case because it seems like it’d be too obvious and the book isn’t even halfway through yet, I figure you could probably at least come up with a workable hypothesis for how she + one accomplice could have carried out the murders.
I don’t know why they just ignored the robot for the most part either, but if it’s dexterous enough(or has enough time, which it probably did) it seems like she could just have written the messages herself, and then made the robot kill her(since it could open the door I assume it would probably also be capable of holding stuff, sending out the robot, etc.). Putting on the wedding dress would probably be too complicated but she could have done that herself. Then it could have cut off the limbs and disposed of some evidence and later sent out the other robot with the corpse on it and rushed right back inside to lock itself inside the room again after, and the “murder” would be successfully accomplished without anyone entering the room.
It could also work out motive-wise since it seems like Magata Shiki’s sister would be the one that could “benefit” the most from her death, so since Magata Shiki already doesn’t exactly seem entirely rational I figure it’s possible she could have seen it as a(somewhat twisted) gift to her sister or something like that(though even without that potential motive the chapter also happens to contain a comment on how meaningless jokes apparently are the best ones and to me Magata Shiki seems like the kind of person that could potentially commit an overly complex murder just for the same of an utterly pointless attempt at a “joke” so you’d have a second potential motive there too.)
She also talked about a doll having killed her parents in the first chapter if I recall correctly, and according to the conversation near the end of the chapter I think the doll was apparently called Michiru. I think one of the messages also said something about needing to leave a Michiru behind as well, and the robot was also conveniently enough indeed left behind, so it would match the murder from fifteen years earlier as well(with the robot standing in for the doll). Aside from that it also checks out that she’s supposedly a genius and that the source of the errors that happened in the lab isn’t obvious to the other characters, and she seems the likeliest to be able to accomplish something like that with how the book has portrayed her so far.
The main problem would be that an accomplice would be needed for the second murder, but there are plenty of people that could have done that as well. I guess basically everyone except the people that were with Saikawa and Moe while the murder happened could have done it. I also don’t really know what motive an accomplice could have with what’s been stated about the overly many characters so far(it’s not like I remember everything either because there’s just way too much information being given to really keep track, like for instance I could have forgotten something that makes my potential murder plan outlined above not actually work out).
(I guess it’s possible and probably even likely that most of this is wrong, but I figure it’s more fun to write it out every chapter before I continue reading and see how wrong I was and how what I thought about the case changed than to just wait and keep reading.)
In other news I think I’m finally getting the hang of who the characters are and how their names are read! (only took almost half the book )
You’re making valid points though!