I am afraid what you are describing is bad news; I have a call into a friend on that.
Had it been Anchor-worm, it would have been obvious. Both pulling (would have been difficult) and the “worm” would have been crusty, crunchy, since an anchor-worm is a crustacean, not an annelid.
My instincts are that it is a single infestation. Though the amount of organic material present it complicates things.
I think you can prepare the Fenbendazole with gelatin, what you want is enough gel to bind the food and Fenbendazole, grind the Fenbendazole as necessary mix at (about) 10 parts food, 10 parts gelatin, 1 part Fenbendazole (10:10:1).
I generally mix the Fenbendazole in the gelatin when it is about cool, then the food, I do not think it real matters though.
What happened with the PP? Did you ever get to 4-hours?
If you pick up rocks or dig around in the substrate, do you or have you seen any mini-earthworm looking things?
Have you fed this tank live food?
Any live Tubifex worms or something you might have thought was something else?