I haven't finished reading the LN yet, but this is a kind of story where the MC is trying to preserve the original plot so they will be able to enjoy watching their OTP from the shadows, not realizing that just by their presence the story has already been changed. And in the next chapter we will...
Hate to be that "ackschually" guy but you do realize that the manga is serialized in a magazine called Monthly Comic Alive that comes out every month on 27th?
If you mean the LN, then buy it on bookwalker or find out there, the first volume should be available.
If you mean the WN, then here: https://kakuyomu.jp/works/16817330666455858898
Provided that you know Japanese, of course.
This is of the stories where it's mostly pointless. Even the author edited the TS from the WN title.
Light novel? No. Web novel? Yes. No, it does not have one. Such is life.
Reposting the map in the comments since it's spoiler-y.
I decided to change the names of the two countries because I felt they will fit better in my opinion spelled like that.