I can't answer your questions about gut loading or whether the red wrigglers are bad for frogs with any factual knowledge. However IMO, from casual reading of posts here and other sites, I've never seen any stating that red wrigglers are bad. To me an earthworm is an earthworm. Just the different varieties of them have a slightly different habit and habitat. Generally, I think red wrigglers are considered a composting worm as they tend to stay shallow. Whereas night crawler's tend to stay deeper in the soil. But as far as their analysis for providing nutrition to a pet, likely not much difference.

In fact, the nutrition analysis for earthworms vs crickets is not too much different with the exception that earthworms in general have a higher calcium content and some consider their ratio of calcium to phosphorus more "ideal". And although I've never weighed them, the earthworms I've got under my compost, many of which are red wrigglers seem heavier than a similar sized cricket that I might give my FBT's, so that feeding of earthworm is giving them a bigger meal for more nutrition.

That being said, crickets are still the main source of food for my fbt's. Perhaps the thing about red wrigglers being bad for frogs was more talking about a specific frog. Possibly the digestive tract of some of the tiny frogs can't handle the aggressive wriggling of the red wrigglers.

As for gut-loading your worms, I suppose it's not much different than crickets. Just put material in your worm bins that is high in the vitamins and minerals you hope to pass to your frogs. Eggshells which I don't think work for crickets, do work for earthworms as they ingest the broken up shells to use as grinding material to help digest other food. And eggshells should be a big source of calcium.

As for the hookworm problem, I doubt that is an issue but it should be researched. It seems a little outside the cycle of infection. But maybe not.