It could be easily done just make a global script that corpses get placed after some ours with decaying version there for you only need 2-3 versions of it would be enough and just replace them wouldnt be much work, and could be easily addet with mods later i guess. Would only be to much work if you have to make a version for every animal, deadra etc but npc would be fine and enough animals could just despawn.
Maybe, but it would also cause lag, as is the case with the permabodies in Oblivion and Fallout 3.
This is why in many games the bodies will disappear so it doesn't tax your system with additional information.
I didn't quite get it why in Morrowind they put in a system which allowed you dispose of the body, where as in oblivion they took that bit out. So you have no way of avoiding the guards. '
There was also a technical reason to do this, bodies and open doors caused lag because it introduced new changes to the environment.