On (1): Different people have different reasons to be vegetarian. Besides the ones mentioned, there are many others. One important one, nowadays, is an environmental concern. Animal farms emit enormous amounts of greenhouse gases; they produce large amounts of pollution; etc. It's also true that animals raised for slaughter are fed a lot more protein (and other foodstuffs) than they will ever produce. They are, if one wants to think of them this way, very inefficient food factories.
Regarding the latter part of (1), obviously this depends upon one's reasons, but most vegetarians I know would never carry an alligator bag.
On (2), I'm not sure I understand the question, but perhaps the point is that shrimp, crabs, and insects do not plausibly suffer. If that is the point, I don't disagree, actually. If one's reason not to eat chicken, say, is that chickens are intelligent, sentient creatures, etc, etc, then this reason certainly does not apply to scallops, or shrimp, so far as I can see. There will be difficult cases, where we do not know what to say, of course, but those cases seem pretty clear to me. Even still, though, one might have other reasons not to want to eat those sorts of animals. The harvesting of scallops, for example, as it is generally done commercially, typically causes a good deal of destruction to the seabed.
On (3), this sort of question is difficult, in large part because humans are the ones responsible for the changes in question. But one might wonder if there are not other alternatives, such as attempts to re-establish a natural eco-system. In Massachusetts, for example, some natural predators of deer have been successfully re-introduced in recent years, and they now helps prevent over-population among our local deer. I just saw a coyote in my backyard the other day! Fortunately for them, our deer weren't around at the time.