I'm not really sure what's killing your snakes.
There could still very likely be some nutritional deficiency at play since just the trout fillet / muscle isn't going to be very balanced, even with supplements. There are also parasites that encyst in fish muscle tissues, maybe some of these have survived and infected your snakes.
I've found T. elegans to take to mice readily. I have two that I've reared from neonates on a mouse diet that are doing very well. With mice, your snakes will be getting a more balanced diet since they will be consuming all the bones and internal organs in addition to the muscle tissues. Since mice are captive bred there is also a much lower chance of introducing parasites.
If yours don't take pre-killed or frozen thawed pinkies right off the bat, rub them on some trout and if that doesn't work stick a little bit of trout in the pinky's mouth. At each meal, offer unscented first and if they don't take it, scent it again. Within 2-3 meals most should convert to unscented. When feeding mice, a weekly meal is sufficient. If you have a bunch of babies to feed, I'd recommend finding an on-line supplier and ordering some appropriately sized frozen pinkies in bulk (they're pretty reasonable).
I got one of my T. elegans as a baby, just barely over six inches and not as big around as a pencil. His first meal with me was a new-born pinky. Even little guys can handle mice fine.
Good luck,
Alice