The thing that bothered me with sneaking was that even after shooting an enemy in the head (right through his ears ) he was still walking and attacking, I believe an attack like this should be fatal, REALLY fatal. One should die from a head shot.

Secondly, I understand perhaps how attacking someone from behind won't always result in an execution but if someone is sleeping it's quite easy to just slit their throat and kill them...

Is it possible to fix these two things with a mod?

Thanks!

A mod for oblivion called "deadly reflex" implemented a sleeping kill animation so it should be possible to do in Skyrim (provided that such animation is created that is).
As for your first request, you basically want locational damage, which is something that oblivion modders also managed to pull off. Wait for Duke Patrick archery mod for this one.

Most of the core stuff from Fallout is still there, but heavily modified, I don't see why it wouldn't be possible to upscale the damage on a particular body part, should actually be fairly easy. Although for a sleeping kill you might want to have an animation for it otherwise it'd look quite odd.