Not exactly- you'd have to separate guaranteed protections (?oProtect) and percent based kill protection (enchant armor). Guaranteed protects are checked first and lost at one per kill attempt. Percentage protection is checked next, and does not expire when triggered.
Example A: A and B both ?oProtect C. C would have to have three kill actions target him to die that night.
Example B: A has been previously been hit with Enchant Armor. B targets A with Grayswandir (a double kill). A would have to pass the 25% chance both times to not die.
Example C: A has been previously been hit with Enchant Armor. B targets A with protection. C, D, and E target A with kills. The first kill triggers the protection. The second kill gets checked against the 25% protection, and the protection saves A. The third kill gets checked against the 25% protection but fails to be protected. A dies.
Example D: A, a warrior, targets B with a kill. B has been previously been hit with Enchant Armor. C protects B. D blocks A. A makes two separate 33% rolls to ignore the block and the protection. They both pass, so all protection is disregarded. B dies.
Is that clear? I believe the only percentage based kill reduction is the Enchant Armor effect, and it doesn't stack.
EDIT: Enchant Armor is 25% protection, not 33%. Also, I forgot Ring of Protection:
Ring of Protection: Kills targetting you have a 50% chance to fail
I need to think on that one- as written, I interpret that as a completely separate roll, as it targets the action itself. That may need to be reconsidered.