I'm not sure specifically what the'sama sama letter day story' is about as it's not a widely known common story. It could be a very personal or a story from a specific local or cultural context.
I don't know who Ken is specifically. The story he is telling could be about his personal experiences, like a trip he took, or it could be a fictional story he made up, perhaps a fantasy adventure with magical creatures.
I'm not sure who Mr Ken specifically is in this story without more context. It could be a character created by an individual author, perhaps a local or personal story where Mr Ken is a neighbor, a friend or an acquaintance.
The Kaguya-sama manga is indeed finished. It ended after a successful run, leaving fans with memories of an engaging and enjoyable story. The final chapters tied up loose ends and delivered a conclusive ending.
Yes. The Kaguya-sama manga has reached its conclusion. It provided a satisfying ending for fans. The creator wrapped up the plotlines and characters' arcs neatly.