I think it's funny in a morbid sort of way that everyone's thoughts (including mine) were "damn." Alas, poor Iwata, we hardly knew ye.
The rest of the volume doesn't give any additional info about the murder, so I'll post my thought about Iwata's last "message." I suspect that, by drawing the same picture as Miura before he died, Iwata was trying to say that he was murdered in the same way as Miura. This was helped by Iwata replicating the events of the original murder down to the minute. Makes me wonder if granny/Naomi killed Iwata around the same time as Miura was killed (even though it's heavily implied, I hesitate to say Naomi murdered Miura since this isn't yet confirmed) since the "drawing shows the time of the murder" theory is now possibly out the window. Maybe through additional clues from Iwata's murder, the police (or maybe just Kumai) was able to figure out who killed Miura.
So I kinda get why Iwata drew the picture of the mountains. But if Iwata's theory was correct and Miura's drawing doesn't necessarily show the time of the original murder, then what was Miura trying to say? Still mulling over this.