Do you happen to know of other similar roadblocks
Cellulose is a huge roadblock. Vegetation consumed as food is loosely classified by its "quality" which is a term that denotes both the food value and the ease of digestion. High quality vegetation would be roots, tubers, fruit, tender leaves and
shoots; Low quality vegetation would be stalks, stems and tougher
leaves. Cellulose is the structural material that reinforces plant tissue, so
the lower the quality the higher the cellulose content will be.
Cellulose is digested in a two stage process requiring mechanical breakdown with special grinding teeth and chemical breakdown via symbiotic bacteria in special chambers. Animals that have neither (i.e. All carnivores) are not able to live on low quality vegetation. Period.
The lowest quality vegetation imaginable is the biologically inactive, dried out stalks of grass plants. (i.e. Straw) There's a big, big difference between hay and straw. Horses and cattle will eat straw now and then but even with their special teeth and digestive organs, they can't live on it exclusively.
On this subject, one of the favorite JW anecdotes involves big cats
being fed a vegetarian diet in British zoos during WWII. This overlooks the human element. What these cats were fed was a diet mixed, ground and fortified by humans and
there are no wild forms of vegetation that even remotely approximate
Another problem with the JW belief in an all vegetarian ecology is scavengers. They play an important role in the decomposition chain and they're so obviously beneficial that JW writers sometimes forget themselves and call them, "Jehovah's natural sanitation squad" which is an implicit acknowledgment that all animals were not "created" as vegetarians.
Another problem is the fact that herbivorous animals are predators themselves inasmuch as plant populations are subject to intense predation from grazers. (i.e.There is a direct relationship between grazing intensity and tree replacement in forests.)
The biggest single roadblock IMHO is that the JW belief in an all vegetarian ecology is a repudiation of the creation argument. (Unfortunately, you won't get much mileage out of it because JW's don't really understand why they believe what they believe...)
Anti-reductionism is the heart of all creation arguments including the JW flavor of creationism. Complex things like flight, they say, could not possibly have evolved because of the sheer number of simultaneous change to various parts of the body that would be required.
Well, the same argument applies to predation. Note how the French naturalist Georges Cuvier described it:
"That the claws may seize the prey, they must have a certain mobility in the talons, a certain strength in the nails, whence will result determinate formations in all the claws, and the necessary distribution of muscles and tendons; it will be necessary that the fore arm have a certain facility of turning, whence again will result determinate formation in the bones which compose it; but the bone of the forearm, articulating in the shoulder-bone, cannot change its structure, without this latter also changes. The shoulder-blade will have a certain degree of strength in those animals, which employ their legs to seize with, and they will thence obtain peculiar structure. The play of all these parts will require certain properties in all the muscles and the impression of these muscles so proportioned, will more fully determine the structure of the bones."
The JW's can't have it both ways. They can't claim that evolution is impossible using Cuvier's same basic argument while simultaneously claiming that predators exist today because, "..existing features were put to a different use.." than what God originally intended.