How? Why? And above all, why hasn't Rum taken care of Shinichi/Conan and Shiho/Ai yet? If Rum has "I took APTX and lived" in common with them—and given that Rum has seen both of them more than once (781–786/652–655; 847–849/731–732; 918–920/814–815)—then it should be simple to do away with them. The BO should already be aware of "our perfect crime weapon gives people a chance to survive," and, thus, should have already hunted Shinichi/Conan and Shiho/Ai down—either they'd found this out when Rum survived, or when Rum told Anokata after seeing them and deducing their identities. Rum should know what Shiho looked like at age 7, right? Shinichi's 7-year-old appearance wouldn't be too hard to uncover, either. And any Vermouth-esque motivation on Rum's for not telling the BO about them, in spite of knowing their secrets, is going to be lambasted.
The fact that Sakurako has had so much exposure to Shinichi/Conan and Shiho/Ai (especially Shiho/Ai) is a major strike against her being Rum—it's the same reason Rumi has a major strike against her being Rum.
On this part I strongly disagree.
Both are officially dead. So even if Rum meets them in persona, which might have happened already, she has no reason to assume that a kid who maybe resembles
another dead person actually is that person.
Japan has a population of 130 mil. So it isnt really too spectacular if one resembles another.
Speaking of Ai we can assume that Rum, due to Ais membership in the bo, knows how she looks like.
Still Ai is dead for the bo, so there is no reason to be cautious.
Speaking of Shinichi I doubt that Rum knows about his look as a kid. And up to this chapter he was officially dead too.
I doubt that any smart/strange kid would instantly would be considered as a APTX-victim by Rum.
But hell yeah, we already have first doubt (CH 783 P7) about Conans true identity, which lines up with the current chapter.
And up to this point Kuroda and Rumi saw them aswell. So if you follow that logic you kinda have to dismiss them aswell.
(And I would have to do the same with Yoko, Higo, Azusa and Sakurako).
That leaves us Wakita (which is an obvious bo-member) and Chikara.
Might be right, well see.
That's called DC pacing—it makes things look as if something will go nowhere, when it really is a matter of just how long it will take for something to go somewhere. Maybe it's just a matter of time until Sakurako's importance is revealed. Because I, too, believe she's more than just some recurring maid character who happens to be Kazunobu's and Naeko's childhood friend... I just don't think she's Rum. She's not the Tomoaki Ariade of the Rum arc—she's the Eisuke Hondo of the Rum arc (as in, she's connected to an important character of the arc, not someone replaced by a BO member... and she was replaced, according to you, right? Is Sakurako Yonehara a real person, or a fake identity made up by Rum?)
I kinda disagree and agree with you at that point.
Your right with this being Goshos (somewhat slow) pacing-style.
But as I said usually every appearence means progess of the overall-story in some way.Kuroda
First app -> Introduction
Second app -> He moved to Tokio, is now near Conan
3rd app -> He gets to know about Rumi being near Conan
4th app -> He meets Rumi
5h app -> He knows about ShinichiRumi
First app -> Intro
2nd app -> Wants to meet Conan or Ai, Wakita and Kuroda hear the news about her
3rd app -> meeting with Kuroda
4th app -> She knows about Shinichi
With Wakita pretty much the same.
I still dont see any other logical progress or context which she fits in except "She is looking for Mary".
This does not need to mean that she is Rum but so far to me this is the most appealing alternative.
And since she is 23 she obv must have took APTX.
If we assume that she took APTX in the case 17 years ago her age in the flashback of Naeko would fit right in.
Its even possible though that she in reality is her mother which died to an, so far non defined, illness.
Unfortunatelly I cant answer all questions.
All Im so sure about that I would bet a lof of money on it is, that neither of our 3 main suspects is Rum.