I don't know how but i have lost the undead cell key and i cannot get in the undead cell.
Is there an editor which i can use to tweak the inventory? The ones that i have found don't do inventory tweaking.

I have not seen any inventory editors. Sorry.

SPOILER

The room contains a pretty cool item. the magicians boots (+2Dex / +2 Cha). If this item is not required for your party skip the room. If you have a Sorcerer, you will miss out big time