
Yashiro possesses the memories of his past life without knowing whose memories they are, or even if they are memories. Fragments of his past self, Yosuke, live inside Yashiro. He thus writes the novel unconsciously. In the scene you mentioned, upon seeing Mikumo's name in the magazine, the main character realizes Mikumo who appeared in the novel Yashiro wrote was a REAL person, too, and guesses that Mikumo and Yashiro were their past selves.
The name of the cat doesn't really matter, I think. The cat's role is to show when Yosuke switches with Yashiro. The cat loves Yashiro, her owner, but shows a dislike to Yosuke. So when Yosuke switches with Yashiro that morning, the cat screeches.
They died because of a coal mine explosion. It's heavily implied. It's also implied, imo, that Mikumo covered Yosuke when the explosion happened, but they both died anyway. (After Yashiro shielded Yoshitake from the falling tools, he said that he was glad because it was his turn to cover Yoshitake).

My guess is they found each other in the tunnel (I fucking hope) but it was so dark they couldnt see each other faces. My guess is they died due to landslide or lack of basic necessities (food, water, etc)
It was the post war era so not much advance with technology, so I think there's no proper machine and tools to dig up a hazardous tunnel and save people
I think I am missing something here....
I feel like I didn't really understand what they're referring in the scenes that seems important....
Like like the scene before when yoshithake ask yashiro is he believe in past life...what exactly happened there ...
Is yashiro starting to write that novel just as a coincidence?
If so why is he named his cat as mikano?
And who exactly wrote about the story of their past life(I mean that old magazine).?
Most importantly...(even though I guess it's cuz an accident..no?)how did they dead?
.
Someone please tell me...? I know I am an idiot who thinks a lot about trivial stuffs in a story i like...but I can't help it..