He is the only one who had a proper chance to put the poison, and whose hindrance, his bad vision, actually can be bypassed.
Morooka Gunzo was far away from the victim's glass and has an alibi (covered by Kogoro and his bodyguards).
Murakami Sana doesn't have an alibi for being far away from the victim's glass.
However, Conan picked out something different with Sana after the alarm was stopped, in comparison to the other girls during the alarm. When Sana reached for her phone on her back, she used her left hand, while the other bunny girls and Yuri reached for their phone with their right hand.
With that detail in mind, Conan will probably remember that Sana wasn't among the bunny girls who were close to Yuri's glass during the alarm.
By making sure his glasses broke and having Morooka (his master) leave to get the spare, Fukamachi was able put in the arsenic (poison) without risking being seen by Morooka, who was sitting right next to him, and at the same time create the defense that his bad vision would hinder him from committing the crime. After setting off the alarm from his phone, he was able to make Yuri (the victim) put down her glass, in order to check her phone, so that he can put the poison in her glass. Yuri's glass was different from the others, but the way Fukamachi was able to tell which glass she was holding is most likely as the hint, that helped Conan and Amuro solve the case, suggested: by using a magnifying glass in order to look for Yuri's unique lily-patterned nail wrap.
in this case, the object acting as a magnifying glass can only be the thing that Fukamachi was holding right before he set off the alarm, his phone (as suggested by mycroft from discord). By using his phone's camera's magnifying glass function, he can zoom in on the Bunny girls' hands and identify Yuri's hand and glass through her nail wrap.
Fukamachi's motive is most likely Morooka's health.
Morooka probably has anemia, deficiency of red blood cells to carry oxygen through his body. Symptoms of Anemia is normally dizziness and loss of balance, which explains why Morooka fell and hurt himself a lot. There are different types of anemia, and Morooka's is probably Pernicious Anemia (vitamin-deficiency anemia), in which there's a lack of vitamin B12 and vitamin B9 (two vitamins needed in the produciton of red blood cells). Symptoms of Pernicious anemia also include lack of appetite and weight loss, which would explain why Morooka rapidly lost weight. If he is taking medication, then it would explain why he started gaining weight.
This makes sense with why Amuro started to remember his past with Elena and how he connected it to Mooroka's problem. There was a lot of emphasis on "red blood running", which could be Gosho's clue to how Morooka's problem is deficiency of red blood cells.
Since Morooka always spent his spare time drinking in the Black Bunny's club with Yuri, his favorite hostess, Fukamachi was probably worried over his drinking, because alcohol abuse contributes to the lack of vitamin B12 and B9.
In order to make him stop his visits, he tried to bribe Yuri to stay away from him, but she didn't accept it.
The old-fashioned threat letter, that threatened Morooka to stay away from the Black Bunny's Club, was most likely made by the old Fukamachi, in order to push Morooka away from Yuri, but after Morooka called Kogoro to solve the problem, Fukamachi went with his last resort, which is to kill Yuri (before she indirectly kills Morooka).
The reason to Yuri not accepting the bribe is most likely because Morooka is her biological father. After Morooka lost his wife half a year ago, he met/reunited with her in the Black bunny's club and started visiting her frequently and drink irresponsibly to the point of getting pernicious anemia. This is why they have a similar eating habit and why he started getting informal with her by removing "-chan", as well as why he got extra angry when Kogoro groped Yuri.
Morooka was together with another woman before his wife in Tottori for 4 years, and most likely got Yuri through her before being forced to leave her, hence why Yuri is also from Tottori. Since Fukamachi was a butler of Morooka's wife's family, it makes sense why he wouldn't know of the fact that Morooka got a daughter from his first partner.
Yuri survived the attempted murder so that Fukamachi could be forgiven, since all of this was just based on a misunderstanding.
Akai vs Bourbon:
Since the Miyano visited Dejima's house "about 20 years ago" according to Ukihiko (File 423), and they were seemingly already BO members, since they were under BO surveillance, it suggests that their visit happened after the recent flashback (File 1011). Since Elena was 3-months-pregnant with Shiho during her meeting with Rei, it can only mean that both the Dejima visit and the flashback happened 19 years ago (which is still close enough to Ukihiko's estimation).
If one Judges File 423 through one perspective, Gosho seems to have retconned things, as he initially implied that Atsushi left his house to Dejima to move to a bigger research facility (which was thought to belong to the Black organization by Conan and Haibara) 30 years ago, but now seems to have moved in the BO's institution 19 years ago. However, It wasn't 100% clear whether the reason Atsushi lent the house was due to joining the BO or whether Dejima was simply recalling Atsushi's institution moving separately.
Something important that was revealed is the fact that the BO were referred to as "Karasuma Group" (File 1011).
In the latest Boss-related interview, Gosho denied that the Boss' name reveals the BO name:
It makes more sense that "Ooguro/Daikoku" (which appeared in two BO-connected cases, File 116 and 281) is the BO name. The fact that Gosho revealed a name that is "Karasuma Group" in the flashback to 19 years ago leads me to believe that the BO changed their organization name after the Kohji case failure. This would mean that the BO was aware of not only the "ASACA" interpretation of the dying message, but also "CARASUMA". Since the Karasuma Group also had bad rumors surrounding them, it makes sense that they would change their image and branding to something else to stay under the radar once more (especially after their name was potentially left as a dying message).あと、組織の正式名称と深い関わりが…と、インタビューでお答えになっていた…という情報を見たことがあり… 出典が定かではないですが（汗）
Interviewer: Also, that it’s deeply related to the Organization’s official name… Or so you answered in an interview… I’ve seen such information… But the source isn’t clear (sweats)
青山 なんだって？そんなの有ったっけ… 組織の名前？そんなこと言ったかな…
Aoyama: What did you say? Was there such a thing…? The Organizations’ name? Did I say that..?
Interviewer: Have you decided on the Organization’s name?
Aoyama: Yes (beep-beep) is (beep), see.
Aoyama: Yes, true, but the boss’ name…? I think it’s not too related…??
Wakasa Rumi's vision:
This could very likely be Gosho seeding in the truth behind Rumi's suspicious head-bumps through this visual cue. This backs up my theory about Rumi wearing strengthened glasses that messed with her perception of distance.