You get side stories from another reincarnators point of view and also kumoko's point of view, but later on realize that what's happening on one side is much further in the future. So essentially you get an excerpt of some future happenings from one perspective and then everything that happens to get there, from kumoko's perspective and then the timelines converge.

The Holy Wars and Transcending the System Arc. Here after Kumo escaped The Great Labyrinth gets caught in a holy war. After a rampage against everyone involved, she is attacked by the original demonic spider. Which spurred her into wanting to be stronger. To which she evolved into an Arachne and eventually beyond the system of the world.

The Demon, Human, and Elven War Arc. Once Kumo-Chan reaches a state of godhood she decides to help the "Demon King" Arale aka "her progenitor" aka "the Original Demon Spider" in her efforts to lower the population of the world and hunt the leader of the Elves.

Kumo-Chan is the MC believed to be a girl reincarnated into a foreign world with heavy RPG elements. After surviving numerous near death experiences in the Great Elro Labyrinth grows into a powerful but lovable monster. Along the way she discovers numerous helpful abilities, including magic, and dividing her mind into separate personalities with each in charge of her different functions (body, calculations, and magic). During this time she meets a armored god speaks to another mysterious god. This will change the course of the very world.

Once free of the Labyrinth, she is mistaken as a servant of the goddess by a small city. After an army starts wiping out her followers and attacked her with powerful magic, she goes into a blind rampage and wipes out both armies. Only afterwards she is nearly killed by her (technically grandmother) progenitor Arale. Who though of her as a glitch or mutant from one of her numerous spider clones. By moving her consciousness into an egg she layer far away Kumo-Chan narrowly survived. She continued to grow and evolved into an Arachne (spider person) and eventually absorbed vast amounts of MA energy she evoked beyond the limits of the system into a goddess.

Upon waking in her new form she is greater by her progenitor Arale. Who ask for help in a plan to save the world. Which has been dying slowly for the last 1000 years. To save it they must pit the demons and humans against each other and lower the amount of life while using the life force of those who died to repair the world. To which Kumo-Chan agrees. And changes her name to Shiiro upon becoming a demon general. Joined by the dragon god, who becomes a fellow general, they lead the demon race to clash with humanity. Meanwhile the Elves are revealed to be a shady race. With a dark history spanning the last 1000 years.

Late into the war we find Shiiro leading the demons on a war path to the elven homeland. A secluded forest guarded by a mighty barrier. It turns out that the elven leader was the cause of the world's decaying. He was formerly a human scientist that lead in the world's bio tech fields. And was the one to discover the world's life force. He learned to convert it into energy as well as channel it into humans and actively evolve them. The result was the planet being s**ked dry and many humans evolving into the long lived elves. The dragons who generally stayed to themselves had enough, and stole the remaining energy and left the planet for dead. The angel Sariel who watched over the humans intervened by becoming the heart of the system. The goddess of the reborn world under the direction of "The Evil Goddess D" (Who spoke to Kumo-Chan in the Labyrinth).

It turns out the leader of the elves also continued destroying the world further. He sacrificed a vast amount of the world's life force in an attack on D. The result was the death of everyone in the classroom where D hid her divinity and played human. Thus she reincarnated all the humans involved plus the tiny spider that lived in the classroom into the other world.

Kumo-chan's story starts nearly 9 months before anyone else is born. And later their stories intertwine in the demon, human war.

Except the ending.

How much did it cost the autor after more than 300 chapters to make 2 or 3 or 4 chapters for the epilogue?.

It could have had the ending that it has, but with more things, all the drama of the last battle, the life of a bunch of secondary, summed up in "and now they'll manage as best they can."

What happened to the teacher and the reincarnated extras, what happened to the hero, what happened to sophia.

Everything stayed in "and now they'll manage as best they can."

I don't see a single reason why a character that already died in volume 2 deserves a volume dedicated only for him, even for character development. I mean, an entire volume just for a character that already died 9 volumes ago???