Rules for Solving the Final Mystery

The rules will be the same as before. All magic spells used (and their properties) have been explicitly mentioned or used within the story. All methods and possibilities for the solution have been hinted at or presented earlier.

Also, below is a list of all of the truths and lies used. I would use these for reference, as a checklist for testing theories until you find one that works, seeing as some vital clues are not mentioned by truths or lies. They have been categorized into general (which apply to everything), and ones that were used in the context of each mystery. If there is any confusion about lies and truths, feel free to ask, and I'll answer to the best of my ability in the comments.

General

“I didn’t replace anypony you know.”

“I’m offended, Twilight! I didn’t replace one of your friends!”

“I am thankful for your help, Cadenza. Twilight Sparkle, only one of your friends was replaced.”

“Your friends refer to Rainbow Dash, Rarity, Applejack, Pinkie Pie, Fluttershy, Cadenza, and Shining Armor. I will also say this next sentence to avoid confusion. Changelings will be included as ponies if any changelings are involved.”

“I do not count as whatever race I am disguised as.”

PAGE’S DISAPPEARANCE

“Your barrier spell lied to you. I changed forms inside of the hotel and skewed the results!”

“Nopony inside of the hotel changed their forms.”

“Nightowl did not put Page in the closet. I did not throw Page out of the window. I shall also confirm the accuracy of your barrier. Page passed through a magical barrier three times, and all such barriers were created by Twilight Sparkle.”

“When I speak of myself, I refer to myself in any form. When I speak of throwing, the term constitutes picking something up and sending it through the air. I shall rephrase my earlier statement, but it still stands. I never forced Page to exit room 201 in any physical way.”

“Magic counts as forcing Page to leave the room in a physical way. I did not use magic to remove Page from room 201.”

“Page did not leave the room intentionally.”

“Page does not sleepwalk. Page was unconscious when she left room 201.”

“Caesura and the pony with her stopped at the second floor on their way to the lobby.”

“Caesura can teleport.”

"Caesura can teleport other ponies."

“Caesura and the pony with her separated on their way down to the lobby.”

“I can teleport while in the form of an earth pony.”

“You saw every single pony that was inside of the hotel that night. There were no ponies, changelings, or other creatures hiding in any other rooms.”

“No more than 28 different ponies were ever inside of the hotel on the night of the disappearances. Nopony passed through the outer barrier more than twice.”

“No more than 28 different ponies were ever inside of the hotel on the night of the disappearances. This does not mean 28 at the same time. Leaving and entering the hotel again would not change that number. Changelings, regardless of their form, only count as one. Cadenza and Shining Armor as excluded from that night, as it was technically morning when they arrived. I shall also add one more addendum. Between when the ponies in the lobby left and when the outer barrier fell, the number of ponies in the hotel never exceeded 11.”

“Rainbow Dash threw Page out of the window!

“Applejack threw Page out of the window!

“Fluttershy threw Page out of the window!

“Rarity threw Page out of the window!

“Pinkie Pie threw Page out of the window!

“Caesura threw Page out of the window!

“Nightowl threw Page out of the window!

“Red threw Page out of the window!

“Preston threw Page out of the window!”

“There were two unicorns in the lobby.”

“I do not count as whatever race I am disguised as.”

“Page was levitated out of room 201.”

“Page wasn’t thrown out of the window.”

“Skylight Dreamer threw Page out of the window!”

LOBBY

“The three ponies that were left behind in the lobby were conscious.”

“I rendered myself unconscious.”

“I rendered myself unconscious.”

“Somepony rendered me unconscious.”

“I accidentally knocked myself out.”

“Caesura did not attend the party. The 11 ponies that Caesura brought in were not changelings. The changelings caused the disappearances in the lobby without help from inside the hotel.”

“There were two unicorns in the lobby.”

“I do not count as whatever race I am disguised as.”

MEZZA’S PETRIFICATION

“Mezza’s statue is real. She was petrified instantly.”

“The Mezza you saw was not a changeling.”

“No teleportation was involved.”

“I do not know a petrification spell.”

“Dolce is a character of fiction. She is dead. She does not exist in the real world.”

“The crime was committed by somepony you didn’t know. Somepony took the form of Dolce.”

“While inside of the barrier, I took no more than three forms.”

OTHERS

Chapter 11: Page: “I did not wake up in the closet of room 201.”

Chapter 11: Nightowl: “I never threw Page out of the window to room 201.”

Chapter 11: Nightowl: “I didn’t throw Page out of any of the windows in the hotel.”

Chapter 14: Red: “I-I’m... guilty.” (The truth of how she felt)

Chapter 14: Red: “I’m innocent.” (The truth of her involvement)

Chapter 18: Skylight Dreamer: “I’m Nightowl. My name is Nightowl. I’m a night guard. I’m a normal pony, living a normal life....”

Chapter 18: Chrysalis: “I will not remove my spell once we have walked 50 steps from this place.”