This is pretty much a copy/paste of the description that I put on Nexus, but I figured that a larger piece like this could do with its own thread. Beats living in obscurity in my old release thread.

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DESCRIPTION:

Systematically removes the essential flags from various NPCs who were essential permanently in vanilla once their related quests* are done. If they die, all unfinished quests associated with that NPC will be failed, and other NPCs should stop speaking about them as if they were alive. NPCs who lived in cities will have a coffin appear in that city's Hall of the Dead containing their belongings.

This isn't quite what everyone is looking for, obviously -- for example, in order to kill Maven Black-Briar, you must progress within the Thieves Guild questline.

Sometimes, who is essential and who is not can be a little silly; i.e., in Markarth, the steward and the Housecarl are essential but the Jarl himself is not.

Obviously, you take a serious risk by letting NPCs the game might expect to be unkillable die. I am not responsible for any lost or ruined content if that happens.

This restores some cut content: if Balgruuf the Greater is slain, his brother Hrongar will become the new Jarl of Whiterun and take his place in later quests.

The name is technically a misnomer; even if you complete every quest in the game, there will still be two essential NPCs: M'aiq and the magic Courier. Everyone else will no longer be essential (or they'll have died a scripted death).

*Of importance. For example, Calcelmo is involved in "Hard Answers" and "The Book of Love"; he will no longer be essential once his part in "The Book of Love" is over.

Some NPCs are no longer essential at all (Rolff Stone-Fist, Bryling, Jarl Korir, Jarl Idgrod Ravencrone, etc.).

REQUIREMENTS:

A working copy of the Elder Scrolls V: Skyrim, and the Unofficial Skyrim Patch. The Dawnguard file requires the Unofficial Dawnguard Patch.

INSTALLATION:

Place the contents of the .7z file in your "Data" folder, which is usually located under Steam/steamapps/common/Skyrim.

UNINSTALLATION:

Enter "SetStage PSNoPermanentEssentialsQuest 100" into the console (sans quotation marks) to make all NPCs who usually are essential again. This will not resurrect them, however; evidence suggests that they will reappear at their usual locations regardless. If not, you can probably find them hanging out in the WIDeadBodyCleanupCell.

You must wait until "sqv PSNoPermanentEssentialsQuest" shows that the quest has stopped.

Then you may delete the included .esp and .bsa file.

Ditto for the Dawnguard add-on, except that the quest is called PSNoPermanentEssentialsQuestDLC1.

BUGS:

As of yet, none.

The script can take over a minute to execute and remove essential tags. Be patient in your violence.

COMPATIBILITY:

Put this at the top of the load order, underneath the Unofficial Patches, because this edits a ton of quests and dialogue.

It also edits Hrongar, Balgruuf's brother, to ensure that he puts up a proper fight if he's the Jarl when the Stormcloaks attack Whiterun.

The new coffins are going to appear in the same places as some coffins added by Arthmoor's "Bring Out Your Dead", but as all the new coffins are new references, it should only be visually distressing.

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Link: http://skyrim.nexusmods.com/mods/25137

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Aw please don't tell me I spent four hours doing "GetDead" checks on dialogue for nothing...

I'm impressed. Been looking for something like this for a while.

Also I'm not sure if there's a way to make the quests failed instead of completed for what you want to do. It was my understanding that failed quests were flagged as such to allow them to be retaken.

Not all of them -- "The Taste of Death," for example, if you kill Eola.