I went into this under the assumption that the mystery would be extremely simple, which is why I started theorizing after briefly reading through the narrative once and quickly rechecking a key point or two. As a result I was somewhat hazy on a number of details, opting on letting the reds guide my way instead of taking my time rereading through the story until I remembered everything properly. With my blues, I decided to focus on a single theory instead of branching out early on because if the mystery really was trivial, the most simple solution would most likely be correct. In responding to your reds, I responded with what immediately came to my mind instead of taking a step back and evaluating things in detail. So, it wasn’t as much a conscious choice to go easy on you, but rather me playing a sloppy game because I was expecting next to no resistance. The clock being past midnight might also have played a part.
I started writing post where I went through the whole thing in detail, but it started turning out way longer than I expected and I didn’t have the time to go through the whole thing, so I decided to cut to the chase and just give you the repeat requests and blues to spare time.
Repeat this in red. It is impossible for living corporeal beings to leave Ritsuka and Robin’s bedroom when its door is locked and the window bars are intact. The window bars stayed intact from the beginning of the story until Ritsuka’s body left the room.
Regarding how the Ritsuka’s body left the bedroom:
This is unlikely, but I want to close off this possiblity for good. Mashu unlocked the door to Ritsuka and Robin’s bedroom before she ran to the attic. Perhaps she did this right after the door was locked by Ronove, perhaps she managed to accomplish this while the four on the second room were splitting into pairs, or perhaps she just plain left Ronove on his own for a while to unlock the door and then returned back to the study.
Also, I’ll restate this in blue for posterity. Mashu left the key in some place where Robin could pick it up and go unlock the door to the crime scene.
Alternatively, Robin chopped Ritsuka’s body into pieces and disposed of them through the window.
Regarding how Ritsuka died (if he wasn’t already dead):
After the door was unlocked by either Mashu or Robin, Ritsuka, who had previously only played dead, went to the attic. There, while preparing to set the stage for the next part of their murder mystery play, he opened the window and accidentally fell through it, dying.
Alternatively, Ritsuka went to some empty room to wait until Mashu and Battler went to the attic. He was killed in that room by either Nitocris or Robin while Battler and Mashu were away.
Regarding how Robin died:
Robin was killed by Nitocris during the night. This assumes Mashu was the one who unlocked the door for Ritsuka to use, which seems very unlikely, but I’ll leave this here anyway.
Robin was either killed by Nitocris when Mashu and Battler were in the attic, he was killed by Mashu when Mashu stayed in Ritsuka’s bedroom after returning from the attic when everyone else had already went downstairs, or he committed suicide.
I’m afraid I don’t have time for anything better than that for now. Any combination of those blues that doesn’t directly contradict each other presents a scenario that could explain the main points of the incident, as far as I am aware.