I have some Creatures that are designed to attack the Player, and ignore all other NPCs-- they do this fine; the problem is, the other NPCs react to the Creatures presence, usually by attacking them or running away in fear. The problem is, only the PC is meant to be able to see these creatures... I've tried raising their Personality, but once the Creatures attack, the Guards and NPCs react accordingly... Any idea how I can fix this? SCAOnActor doesn't work, since it stops them being able to engage the PC, too

It's a long shot, and maybe others will have better ideas.

But have you tried using a script that detects if the NPC is in combat with a creature and then run stopcombat.