Indeed, they completely twisted the story, in the web novel the gyaru sister was the sacrifice, after MC was "invited" and dislike dhow the elders behaved he threw a comment about the younger sister being a better sacrifice as her origins were clear and therefore would have fit best as a god sacrifice, that made her scared shitless on the spot and the elders actually pondered on the idea of switching them that I think they did.
MC accepted to kill the land god after the mother asked it, no need for a blackmail, even if I think the manga version matches better with the initial concept of him living for himself and ignoring such absurd request from a group of people that was openly hotile to him, but forcing it with the mother inviting a perfect unknown girl in their house was a bad take.
As you said the gyaru behaved so that it would easier to select her, and the bratty sister tried to cut her off to avoid the pain, but there was even one of the elder planning some move to avoid the sacrifice, I don't remember now if it was using some artifacts, fight herself or let her escape, anywya, the gyaru wasn't detested by everybody like she thought.
I honestly believe that some changes web novels get in serializations is to justify the editor paycheck, as several times I believe the original writing was already perfect and altering it only lowered the quality, usually with garbage overused cliches (for example Raising of Shield hero, got MC having to run a chocobo race with the Spear hero, nothing like that in the original story).