Freezing kills parasites only if done at long enough temp and for a long enough time and both conditions are pretty strict!
Freezing doesn't kill bacteria (or all bacteria?).
Pacu having been in the tank only for an hour would be plenty enough to contaminate the tank with a pathogen and it is highly suspicious they had died and 2 weeks later the RTC died... but if Crimson contracted some disease from pacu, it'd get sick for a while and it'd be obvious and then die. Since it ate the night before, it couldn't have been a disease.
So we are back at the crappie + bones hypothesis. If you might have missed a needle sharp bone or a piece or a spine etc. and Crimson stuffed itself till round, it is fathomable that its tummy and a vital internal organ was pierced. Still a very rare occasion but possible, especially if the crappie are sizeable, not fry or babies.
If the crappie were small, I'd see no reason not to feed them to a catfish whole. If worried, fin spines could be snipped.