All right AriaGirl. Is it only the sims in your own controlled household that are invisible? If it is so I would recommend moving them with Edit Town (this may work since the problem may be related to your error code 12 - the world-cache bug; often this is related with Edit Town in some way). I'm aware that you're playing a legacy so that may be banned by standard rules, but at least it's worth to see if it works just quickly (i.e. not saving after moving the household and then just re-load if it doesn't work).
PS: Try to recover an earlier save first before you do anything else.