This sounds like a mod. The Countess is flagged as essential.
It's not, it's vanilla. My questlog had already updated and as I had already done the Thieves Guild quest, she was no longer essential even though the quest for Sanguine was still in progress.
Not to mention you can't drag bodies out of cells, so I'm not sure how any of that is even possible.
The door from the castle reception hall and main hall to the dining room is a "wooden door" so it's the same cell. She died in the dining room and her body was dragged all the way to the reception hall, right in front of the main gate, but still in the same cell.
I figured the spell was considered an attack. I dealt with it by teleporting out and running to get my bounty lifted before the troops caught me.
I think if you do it right, they don't know you did it.
Correct, though I wonder what you mean by "teleporting" in Oblivion?