> is it desirable to use the fewest 'animal products'

It's not even a consideration.  It is desirable to use the product which best
produces the desired result.

> is it possible for some people to be healthy vegetarians

No, IMHO

> is it possible for all people to be healthy vegetarians

Unequivocally NO

> what's the difference between a dog and a cow

For starters, the dog is a carnivore and the cow is a herbivore.

So what's your point?