Is it possible to punch someone’s head off

The seal on the ground is bound to react to the Dragonborn blood.

It is automatically activated when you step on it if I remember. It is possible thought that you need a specific quest for it. You should be accompanied by Delphine and Esbern at this time. It's part a a quest series to defeat the main opponent. You are going to Sky Haven to learn about history.

When you get killed by a decapitating strike, a flag gets set in the game state to denote that an animation should be played showing that you got decapitated. Upon loading the game; the animation of the body appears to be overridden by another animation (eg. your character wobbles even though your standing still), so the flag does not affect the body part. However, it does affect the head...

I see two ways to get rid of this annoying stuck flag:

1. Get someone to do a decapitating strike on you without being in immortal mode.

   If lucky, the flag is removed upon death. If unlucky, it just adds and removes a second flag...

   Update:
   The above fix won't work, once you get killed it will just reload the save game again. In a similar vein the other fixes mentioned in this question (and elsewhere) do not work because they do not clear the flag from the save game. Editing the save game seems to be a necessity to fix this.

2. Get two save games, the one right before and after the incident. It might be more feasible to try to reproduce this and get two save games in which there are less changes...

   Then, use Notepad++ to do a regular expression replace to change :: into \n.

   Now you can use WinDiff and set it to show right-only lines. If done properly the amount of changes would be minimal and if you look at the lines containing _var variables you could easily figure out which flag affects this. Replacing one character should yield the flag invalid, do however not add/remove characters as that might break the file.