In terms of purely the personal health aspect, I wouldn't say it HAS to involve being vegan, but certainly even modern "western medicine" accepts that a diet containing over 10% animal products exponentially increases cancer cell growth rates, leads to diabetes, and so many other negative outcomes. If you look at the whole picture of how you are affecting the planet and all other beings though... it becomes a lot harder to justify eating animal products/by-products. The only way you can do it and not fail each of those "tests" is if you are raising the animals yourself, extremely small-scale, letting them live their lives, and for things like dairy/honey, not taking anything but the excess they create.
As to your more specific points:
a) I have never heard an actual case of this myself. I have heard of people's health decreasing when switching to a vegan diet, but all of those were people with unhealthy diets to begin with, who simply switched out meat & cheese for lots of soy, wheat, and processed non-foods. Any "diet" (paleo, vegan, vegetarian, HCLF, LCHF, etc) is extremely unhealthy if you aren't getting the protein, fiber, nutrients, phyto-nutrients, and probiotics your body needs.
b) I don't really have any take on this particular thing. Certainly every herbivore eats some insects, by the nature of insects attaching themselves to plants, flying into mouths of sleeping animals, etc. Personally, I don't eat oysters or any other shellfish, and I don't eat intentionally eat any insects. I do not have the need to supplement my diet with these things, nor the want to eat them. We all live in different places, have different resources & access to resources, and have to make all of these choices for ourselves.
Regarding "A", it's me, I'm the guy! I get extraordinary digestive pain and distress if I try to go on a vegan or even vegetarian diet, despite many attempts.
Thanks for your insights.