i don't know how to help but pointing out the following in trying to zero in on the culprit:
- for a pleco to die from water quality, the water must be pretty toxic. however, if its death was from a kill, the killer must be worse than a jar in temperament. hence it would wipe out everyone else in the tank eventually if it could.
- does it look like slime coat peeling to anyone, the worst kind? well, besides those red markings/wounds. i know this may not be as simple as slime coat peels but it does look like it to me.
- can you separate the silver? i know that's not easy to do, but doing so would tell if the culprit is in the tank mates or not.
good luck and i hope you have enough time to find out the cause and treat it timely.