You really want to know?
From what I understood Mio understood that her son was dead and had not gonne to another world, but she was really, really suffering so much that she wanted to believe this and sometimes she really became a bit delirious plus the suicidal thoughts. I think she knowing the truth what was prevented her from having killed herself despite she getting close sometimes. But more importantly the death of her son wasn't the only cause for such great suffering, the circumstances colaborated a lot. Her son was everything to her, her husband married against her will, was always cheating and arguing with her. She didn't really cared as long as she had her son and he was a really good sound that loved her. Beyond this, when he died by chance and without anyone paying attention she took by herself the responsibility fill the forms about her son death and notification for the funeral. She filled dozens for forms and letters telling about her sound death and this was what traumatized, while she still didn't had processed the information that he had died she felt like she as killing him every time and that's why she was so obsessed with the other world idea. She felt guilty and wanted to meet him against to ask for forgiveness for having killed him.
In the end she tries to kill herself again but with Doubara help she comes to accept everything and vanishes.
In the last chapter she appears again to visit Doubara. His work and life improved a lot after what he dealt with with Mio and during this time missing Mio calmed down and returned to normal. I thought it was really nice the page that showed how she had accept and got used to her son death. She was reading a LN and at the end it said that it would continue the next year and she was loging forward to it. That's when she noticed, "next year", "tomorrow", "lately", "next time", "from now on", "continuing"... she notice that she had all these expectations for the future.
When she meets Doubara again she ask to let her read his novels. He actually wrote a history and put online and she identified it was from him because of the situations and details.
They became true friends and better people.
And that's it.
There's more details about other characters but about the main duo that's the gist.
Sorry if it's confusing.