The way in which Gosho hands his place to Rumi(by twisting words and giving Rumimore screen time.) it's evident that Iori is a big-shot.
Regarding your Iori-thoughts:
First of all: Some example for "twisting words" plz.
I dont quite get that but I try:
So you state that with "giving Rumi more screen time" / putting Rumi in the focus of things
...this tells us that Iori is a big-shot, because he isnt getting as much screen time as Rumi / isnt as much as her in the focus?
I agree with that logic (person not in focus is important) but I disagree with the person you choose.
Furthermore you have to think then that he used APTX / Silver Bullet to keep his youth regarding the case 17 years ago.
I wont dig in this deeper since it will open up again the discussion about him being a main-plot related character.
Other point regarding the dying message:
I think that from a criminal-literary point of view there is pretty much only one option left:
The solution of the dying message presented to us is actually wrong. The dying message isnt Rum = Asaka.
The reason for this that in criminal literature you never will get the (right) solution of a cryptic dying message directly after you the hero
of the story discovered it. But here we have exactly this. Conan and Akai solve it 2 chapters later / even in the same case with the Rum-arc being only 50 chapters old.
So the only logical conclusion is that the solution is actually wrong since this isnt how criminal literature works with no twist being there in the end.
This would also explain the "C not K"-problem.
So yeah, I state that the true dying message is not Rum=Asaka.
Other point regarding of when Gosho actually made up the Rum-arc:
As far as I see this wasnt discussed too deeply yet.
Its obvious that no reader can actually say this for sure since we cant mindread, but I try to explain my logic and point out some possible hints.
To me the main person of the Rum-arc so far is Mary.
She is the one being hunted by the bo and it seems that it will all result in a big clash between Conan&Co vs BO.
So I think that Marys first appearence pretty much marks Goshos first thoughts about the Rum-arc.
But when did Mary "appear" for the first time?
If you look up here you will get CH 875. This appears to be right since about 20 files later the actual Rum-arc starts with Rum being mentioned in the sms.
To me this is wrong.
Mary actually was mentioned the first time in CH 771 P3. [ Sera: But my moms breasts ... ]
I also state that Mary was the one speaking to Sera in CH 800.
(Shuichi doesnt make any sense and I doubt that it is Shukichi too. If you compare it to 1000-1005, you kinda see Sera being in connection with Mary the exact same way via those small headphones)
Its actually remarkable that he let Sera live in a hotel from her very first appearence.
This actually leads to me thinking that he had the Sera+Mary-combo in his mind from the very beginning.
I would even go that far and state that Gosho had the Rum-arc made up by the time he let Sera appear the first time (CH768).
So I think that there might be some possible clues even between 770-900 and not only from 900+.
So if you see 770 as the actual start of the Rum-arc its really interesting to see who is introduced in a 6-chapter-case about 10 files later.
Just some thoughts ...