As MeiTanteiXX stated in the RUM arc post it is very possible that Rumi was Kohji´s bodyguard and at the same time, had a sentimental relationship with him. Amanda couldn´t rely in BO, either FBI or CIA (it is very possible that RUM could be infiltrated in CIA or FBI , makes sense because normally is the good side that have people infiltrated in the BO.
It is very possible that Amanda had a delicate chat in his room about what´s going on (Rum, a terrible member of the BO is infiltrated in the FBI and CIA), she believed that Kohji as a very intelligent guy and she asked for help, Kohji probably thought that Amanda was a paranoical old lady but just for her tranquility he let her his bodyguard (Asaca-Rumi). After all, a Shogi player is not a political figure or something great.
The FBI or CIA (therefore, RUM) could have put on her a transmitter and hear the conversation.
After this...my theory is not completely clear, but I have my own assumptions...:
Rum then prepares a quick plan in which he disguises as ASACA. Amanda shows her gratefullness to ASACA giving her a hand mirror. Possibly because Amanda sees her as a manly bodyguard buy with a feminine side. After that Amanda prepares herself to go to bed and ASACA guards her room. (That´s probably when Gaito Hotta sees her with the mirror). ASACA kills Amanda leaving no trace and then goes after Haneda.
ASACA(Rum) enters the room and soon Kohji notices that something is wrong, they have a struggle fight. Kohji uses the scissors and hits RUM in one of his eyes,removing his mask and splitting blood. Rum wounds Kohji and finally makes him ingest the drug. Then, Rum uses the glass trick to incriminate ASACA (removing probably ASCAAA) while hearing someone is coming he tries to flee.
I don´t believe that Kohji could left that dying message, we saw how Shinichi or Shiho suffered from the effects , that doesn´t give you enough strenght to smash the mirror and all the stuff. (that just could be possible if someone was present too in the room, and even before Kohji takes the drug).
That is the the real ASACA and it´s probable that she knew something about RUM before she bodyguarded Amanda, and it´s probable that she found out that she was dead and then went to talk to Kohji. When she arrives in the room she finds Kohji dead and really a mess, along the hand mirror with a dying message ( ASACA) , knowing that this would incriminate her, she removes U and all the test of words to change the dying message to ASACA RUM, leading to the conclusion that was Rum who killed Kohji and Amanda. She algo takes photos of the crime scene (but I don´t know why she would have a camera) before the police arrive.
The fact that Tsutomu´s blood was in the crime scene it´s probably because he was wounded by Kohji, thus this he had to disappear from
public scene, sending that strange message to Mary. Even though he might still have allies and people infiltrated in the FBI and CIA.
I believe Tsutomu Akai is Rum, he seems to be an intelligent man, mysterious and really linked to the story.
I think it´s very clear thar Rum is infiltrated in the good side, he´s Nº 2 inside BO, and they usually are one step beyond.
The only reason I think the boss didn´t punish Rum after the Kohji Haneda case it´s because he is a fundamental piece inside BO.
I don´t know who could be Kuroda and why he has such interest in Rumi. He could be someone that knows that Tsutomu was a BO infiltrated, he could be Tsutomu or someone related to the case. He is for sure related to RUM and that accident years ago.
Rumi´s existence is clear, she just wants to know who is Rum. That´s why she always ends in cases aside Conan or police members. She believes Rum is infiltrated there. So she is stating that she is ASACA and she is the one that fixed the dying message in order to incriminate Rum, she is the one that is uploading info from the case and she is probably investigating BO since that crime. She wants to make them nervous.
Another thing, maybe the BO believes that Rum could be an old, strong built and feminine from this crime. Tsutomu, in order to hide his identity could play ASACA, Amanda´s bodyguard can reunite this three affirmations. Feminine because the long hair , strong built because he was a bodyguard and old because 17 years passed...But anyway, this is just a weak assumption to fill another hole!
I know this theory has some holes, but not more holes than Kohji´s case had haha.