The culprit is
Ayami Yamaki. Yamaki went first to choose the leftmost cabin and waited for Nanbu to depart to create her first trick : she removed the ribbons from her present package and taped them to a pair of soles she put over her phone in front of the rightmost cabin, to make it believe that cabin was taken by another customer with high-heeled sandals. She then quickly took her second trick out of her bag : a straw whose both ends she had closed with a hair straightener after putting into the straw some chloroform. She sneaked into her boss's future cabin and opened one end of a straw with scissors to pour the chloroform on the first cabin mask of the box, then left as her boss arrived and took obviously the remaining central cabin. As Sashihara put the mask on, she accordingly passed out due to breathing chloroform, and Yamaki entered once again the cabin with a rope to strangle her, before leaving, and retrieving the fake shoes, which allowed Nitsuka to enter the rightmost cabin at last.
She then discarded the incriminating items (straw cut into pieces, scissors, tape) in the bin, taped again the ribbons to her present, put the soles into her own stilettos, recovered her phone, and waited for Nanbu to come back and check if Sashihara needed help, discovering the murder. Yamaki tried to kill Sashihara a week ago but this failed as Nitsuka entered too early the cabin Yamaki had prepared for her boss, and accordingly Nitsuka passed out due to the chloroformed mask, while she thought this was all due to Sashihara's dog's hair to which she's allergic, hence the need for sandals trick to prevent Nitsuka from unknowingly foiling her plan the second time.
Sashihara pointed out Yamaki as her murderer by reddening one of her fancy nails with her lipstick, and showing it forward all the while clenching her fist : the fancy nail patterns are yellow stars, and the red on it indicates China, a country she knew well. She wanted people to understand that in China they count in another way, they can count up to 10 with a single hand. Sashihara's other hand actually shows a kind of gun-like shape, but it actually refers to the way Chinese people show the "8" figure. And it indicates the Kanji for "eight" in Yamaki's name.
Yamaki left her fingerprints on the tape in the bin, proving she's the murderer, she didn't think anyone would find out her tricks. The second evidence is her feet and shoes probably covered in dog's hair, which is weird and means she went really close to her boss, to murder her then.
Yamaki did it because she had been humiliated by the victim when she dressed her dog in the same outfit as Yamaki. Yamaki understood that she was a slave to her boss and wanted to leave the company, but couldn't since she was in debt to her boss.
Conan thinks about how Masumi didn't know the difference between how Americans and Japanese count and how Europeans count, and connects it to other instances of Masumi saying and doing things that someone who lived in America wouldn't—being surprised at how thin a baseball bat was, and keeping her fork in her left hand while eating during the Red Woman case, and referring to soccer as "football" during the Romance Novelist case. Conan realizes that these instances show that Masumi actually lived in the United Kingdom, not in the United States — in Britain, cricket is more popular than baseball and has a wider bat, when eating, one would not switch the fork to the right hand, as is the case in the United States, and the British refer to association football as "Football", while in the U.S. it's called "Soccer".
As Conan wonders why Masumi would lie about this, he begins to think of England, and finally remembers.
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