kzomo

https://www.fmgmh.com/comic/xueyuai.html

From what I could follow: two of the kids from shows era were in love. Post War era (the modern industrial era where they have those mining companies promising better livestyles with accommodation) . Instead of going for university they decide to work in coal mines. One of them is a writer (black haired) in school newspaper. However, they have this explosion where they work together. And they both die. One of their friends who is playing the piano is that funny kid's grandpa who also is a traditional confectionery shop's owner. (funnily those sweets are also something mixed into the Japanese culture from west after war)

So they don't reincarnate but live their missed lives into the main characters spiritually. Since both of the main characters also love each other. So their relations is at parallel. When they confront each other they feel fulfilled. But they completely leave after the deed :)
Hope I'm not mistaken

Yeah kinda like that. Both use the body of respectively looking alike MC and ML. At some point they leave

Two of the current characters were alive

Thank you so much ٩(๑❛ᴗ❛๑)۶

Yw:)