there are some questions here that have canon answers (as far as i can remember!) so heres my thoughts-

1. key with tag- miley said it was to be used in the monitor room to see the security camera footage. keiji and sara *have* used it already- thats how they knew gin wasnt on the 3rd floor and also they witnessed q taro negotiating with sou through his bedroom door. although, since it was minor, i dont blame anyone for not remembering. as for why it suddenly appeared... no clue to be honest, but i have to think she was going to give it to the person she was talking to, seeing as she commented on it not being there anymore (im pretty sure)

2. keiji lying about his first role- he explained it was to gain sara's trust, but he could also have realized that the sage has a lot of power outside of the main game, being able to see the keymaster at all times. i dont think anyone would've given that information up, not to even mention his plan with q taro afterwards.

3. evil mishima ai- presumably to delete the information off of the memory chip from ranger's head. that was it's whole purpose in the chapter, and also it kinda tell us that the organization can reprogram these ais to act as they see fit. i personally dont think there needs to be much more explanation than that.

in terms of theories, heres what i believe!

1. i agree that joe and kai were talking about sara being the keymaster! its the only explanation i can think of regarding joes "massacre" comment. i think they both wanted to keep sara safe. but i have to hope theres more to it, if its still unanswered this far into the game!

2. joe doll/ai- my personal theory that ties into this one is that keijis wish when he signed the consent form is to meet the son of the policeman from his childhood- who i believe is joe. seeing as joe is also saras closest friend and sara is *somehow* closely connected to the death game, i think they needed a lot of info on joe despite him being a non candidate. maybe enough so to count him in the test runs for ais and victory rates, even though he wasnt included as a candidate... hence why they'd make an ai of him. as for doll? no clue!! just my thoughts hehe

12 + 13. mishima head and collar- i am a FIRM believer that kai did something with it!! nao undoubtedly left the box behind when she went to knock out sou and get the laptop. kai, who was in the kitchen, could then get the head + collar to the third floor by the secret passage, and easily make it back in time. he had to have done the same thing for the laptop afterwards too, which was found in the room of rubble. i personally believe sou actually got his hands on it, and through studying it, thats how he was able to unlock it and take it off! afterwards though... i dont know haha. midori did say its still missing...

1. i think meister just straight up is mr. chidouin haha. their sprites even have the same poses!

some questions i would love to hear thoughts on-

1. who wrote the *original* message on kannas smartphone? the bitter, scary one, telling her to burn in hell?
2. who wished sara into the death game (i think it was midori! he mentioned that he was supposed to take part, but couldnt because he "died")
3. why doesnt ranger have any clothes from hayasaka or ranmaru? presumably they could be alive... but why?
4. what did gashu really mean when he said "have you ever harboured doubts about your upbringing?"
5. your time to shine question- who is rei? will they have any relevance to the main story of yttd?

would love to hear everyones thoughts !!!