The show is definitely a mystery. It only no longer becomes a mystery if you give in to the witch and her fantasy. Battler is super-weak right now because he refuses to believe the family/servants has anything to do with the murders. If a fantastical 19th person actually did exist, then they do have magic powers because they couldn't have had the necessary keys. However, once you start thinking that maybe some of the family/servants are involved, it becomes easy to disprove the witch.

Regarding the red truth, the purpose of it seems to be so you can tell some of what actually happened when you're shown scenes that obviously did not happen as shown (no goat monsters are running around the mansion, obviously). If you don't trust the red truth, then there really is no basis for the mystery vs. fantasy debate (and it would be a super lame move by the writer).

Following on with my theory from last post:
At this point, I think Gohda is the strongest suspect for being Rosa's accomplice. He was last alone with Jessica and Kanon, and he probably could have helped Rosa set up the Halloween party as well as poison/kill the siblings.

So, "Kanon" comes in the door. He looked like "Kanon" at first, but he really wasn't. "Kanon" had died and Kanon's true self is now was out for blood to revenge Jessica. I'm going to guess he was pretending to be wounded, but maybe Gohda hurt him in the fight. I'm not sure why he would have attacked/killed Nanjo and Kuma (if that is what even happened), but after that, it seems like Gohda pinned him to the wall and Shannon got something to knock him out with (chloroform maybe?).

At that point, the 3 made up a story to tell the ohers. My guess as to why Gohda is so nervous is because he was supposed to have killed Kanon off. Shannon is probably crying because she is lying to George, and Genji is super-cool about the whole thing. Kanon is probably chilling tied up in some secret room at this point.

I feel like the letter is extra-suspicious, because who would have wrote it? Rosa couldn't have because it mentions the 2 corpses. None of the servants really seem surprised about the letter, and it has to be one of the 3 because of the keys (perhaps Shannon and Genji carried off knocked out Kanon and Gohda secretly did this?).

I don't have enough to piece together the motivations at this point. There are a ton of unanswered questions here, but there is definitely no witch involved. Don't give up so easily!

I'll have to wait to see what happens with George/Shannon before I decide what actually is happening there.