To your specific questions, I have little evidence. I am told that adrenaline will ruin meat, but I have no citations to support that.
Dunno if it "ruins" it, but it sure makes it taste like ◊◊◊◊.
* The physiology of humans was clearly evolved to handle consumption of various food sources: including meat and vegetables, etc.
Just because it
can handle certain foods, doesn't necessary mean it
should. Certainly not in the quantities that most westerners tend to eat them.
* Certain proteins, and other nutrients, are harder to come by in vegetable form.
This a myth that is long since debunked. Oh, and the term you're looking for there is "amino acids", not "proteins". The idea that it's not possible to get a sufficient balance of amino acids has been thoroughly refuted, and was based on an outdated and erroneous concept of how many seperate amino acids are actually necessary (the human body has been noted to synthesize most amino acids it needs), as well as the amount of protein necessary. The one amino acid that was considered tricky is Lysine. However, this appears in moderate concentrations in most grains; and in very high concentrations in soybeans, amaranth, and a few other sources. In fact, soy has a higher concentration of bioavailable protein than any animal source, and amaranth is not much lower than most animal sources. Legumes have all the other necessary amino acids in highly concentrated and bioavailable forms.
The average adult residing in a First World country eats many times the amount of protein he actually needs in a day. A bowl of beans and rice, or a few ounces of tofu, a day is pretty much all that's really necessary. Obviously, if your body is still developing, you're sick, or have one of a number of system dysfunctions, then you're going to need more protein, but that's not difficult either.
The other old myth of vegetarianism is lack of iron. This is also untrue. Dark green leafy vegetables, most legumes, and many root vegetables contain at least as much bioavailable iron as any animal souce. Iron deficiency is the result of a poor diet, not a vegetarian one.
The only serious deficiency of a purely vegan (no animal products at all) diet is vitamin B12. This is definitely not a myth, since there is only one known siginificant source for bioavailable B12 that is not animal-based, and that is a particular kind of yeast. The big problem with this deficiency is that it's a very slow and progressive one. The body contains large B12 reserves, and it can take years for deficiency symptoms to manifest to the point where they're obvious; and by that time, the damage is often done. This is not a problem for non-vegan vegetarians who eat dairy and/or egg products, as both of these provide sufficient B12; and taking regular cultured-yeast B12 supplements solves the problem for vegans.
I'm, personally, a vegetarian for health reasons. I cannot easily digest animal products. Not sure if it's an allergy as such, or some other dysfunction, but I am unable to eat meat. Fish I can manage, and eggs and dairy don't seem to be a problem, if I don't overdo it.
It is possible to be a vegetarian/vegan and be extremely unhealthy; the same as it is for an omnivore. Eating too many processed foods will make anyone unhealthy; and many pre-packaged/processed vegetarian foods are very high in unhealthy fats, due to the amount of coconut oil, palm kernal oil, and hydrogenated oils that they use; all of which contain more saturated fat than any animal product.