My intuition is that your view of animal happiness is closer to the truth than the vegetarians. First, I consider research on happiness even in humans, especially "Stumbling on Happiness" which book I highly recommend. Even for humans, happiness does not generally correlate with what you or I think would make you happy. A compelling example: conjoined twins are generally as happy as "singleton" (i.e. normal) people, but virtually no singleton would guess that intuitively. Generally, we adjust to the status quo across gigantically broad ranges: those who live in the slums of Bombay are not clearly happier or less happy than those of us living in mansions.

So I would imagine chickens in ooky coops, cows in stockyards, like humans, adjust to the mean. Then have their moments of pleasure and moments of pain primarily as variations around that.

And the terror or fear at slaughter? It seems very unlikely that they spend much time dreading it, as my dog trainer said to a couple who was sure the dog was punishing them for going out by pooping on the floor: "I think dogs live more in the moment than that." And I expect that for cows, pigs, and certainly chickens.

So far, we live in only one world of a possible MWI. So far, mammals are born, they live, they experience emotions positive and negative, and they die. How much sense does it make to adopt a moral system which thinks we are wrong for just doing what nature has very many animals do for millions of years?