I suggest you read Peter Singer's book, "Animal Ethics", which goes into great detail on the ethics of consuming animal products within the framework of utilitarianism. Singer is often thought of as the father of veg*ism, so he's a great place to start. I don't know a great deal on the topics of environmental impact or health, but I think I can start some discussion on your questions.

"Can you rank animals by how bad eating them is?"

That depends on your personal views on the sanctity of life or lack thereof. This is a very easy question for me to answer - of course, for humans are animals and I place the value of their lives over others. Similarly I care much more for a pig than I do for an ant, but not so much that I disregard the ant entirely. It makes perfect sense to have a scale for which animals' lives have more worth than others, but I'm wary to do so in fear of being dishonest, since there's no way I'd put myself below top.

"Is it more ethical to eat wild animals because they have a good life before dying?"

I suppose, but it doesn't warrant discussion. This is merely a question of whether the life of an animal in its natural habitat is better than one on a factory farm with killing and eating them as a given. The answer there is obvious and I just reject the idea that killing and eating is necessary to begin with. I also believe a wild hunted human would be a more ethical meal than a factory farmed one, but I believe neither are acceptable. My views on factory farm and wild hunted animal flesh are roughly analogous to my ones on human flesh.

"The ethics of offsetting" (can I offset myself into ethically eating meat?)

The author of the article in question says here that they made some mistakes -

(12/30/16) In Vegetarianism For Meat-Eaters, part 2 suggested that donating to animal welfare charities could save 3 – 11 animal lives per dollar. Based on critiques like those in this essay, I now think those numbers are heavily exaggerated, maybe by several orders of magnitude. I don’t know what the right numbers are or whether the point is still somewhat valid.

That aside, I do not. In order to be ethical agents, we are obligated to do no harm. Regardless of what good you do in the world, you cannot act ethically and also intentionally do harm. Assuming imprisoning and killing animals is harmful, you cannot engage in it and consider yourself ethical in your treatment of animals if you believe that factory farming or meat at all is wrong.

These are of course my opinions, feel free to disagree with them or let me know with any questions or problems you spot.