To make a long story short, I was confused on why Mephala's Quest was so short compared to every other Daedric Quest in the game, so I looked up a quest walkthrough online to see if a glitch made me skip a few steps on the quest. But then I come to find out that, if one is to believe the game files, the quest would've been much more elaborate, featuring Balgruuf's children falling under Maphala's influence and killing him, with his brother replacing him as Jarl. I was just wondering if anybody knows the reason as to why this quest was cut short, and if it would be possible for somebody to make a mod to restore this quest to (what I'm assuming was) its orignal form.

I haven't checked the game file but
is definitely taking things too far. The existence of children is strongly debated in the first place. Some said we need children to add more realism, some against it as to not make the game be branded as children kill simulation. In the end, Beth took the middle road by adding children but making them invulnerable. So I don't think they would include some quests where you must kill a child.

Killing Balgruuf, not his children.

The chlidren murdering Balgruuf isn't much of an improvement.

Actually, it's the player who kills Balgruuf. The children do succeed at killing their caregiver, however.

If I had to guess, I would say it was the rush to make the release date combined with the fact that killing Balgruuf would probably accidentally create many issues with the later portions of the main quest and Civil War. Don't have time to code in a path that allows both those (arguably more vital) quests to continue if Balgruuf's brother is the Jarl instead? Scrap the idea.

I do agree "The Whispering Door" is stupidly short as it stands, though. Does a complete disservice to Mephala, especially. She's supposed to the Webspinner, not "here, go pickpocket a key and open a door. kthnxbye!"