If you notice the two who say they have had success also have triggers with the same body type and diet.
Urchins and crabs are not the first choice of Blue Jaws and Nigers, but for triggers with the diamond body shape IE; Huma Huma, Clown, etc, urchins and crabs are top on their list of goodies.
That is why their eyes are set so far back and the business end of the mouth is essentially a point, so they can get past spines and claws without much risk to the eyes.
I think Blue Jaws and Nigers would be more apt to go for shrimp or other crustaceans that can swim, plus they both also graze on algae more than other triggers. you can tell by their mouth shape/structure.
Not saying they won't eat crabs and urchin's, just that other types of triggers are more likely to.