Instead of a crown of protection, why not implement a new spell of poison protection? or a spell of poison remove?
Consider that everything that have influence on the chance to take a poison by eating toadstool, makes the "I eat dead people" less useful...I have that perk, and, if the chanche to be potioned will lower, My money (and, above all, my pp) that I have put on it will become a not beautiful deal...anyway, this discussion is very interesting!
see you in game!