An issue I'd have with some of the suggestions is that they aren't realistic. If the message is that what slaughterhouses do is wrong, then the game should accurately depict them, not an exaggerated version. For example, if killing animals without anasethetic is illegal, then a game that implies it's what abbatoirs actually do would be manipulative and misleading.
It's possible you might be identifying yourself as a member of the game's target audience. Like Shadowlord points out, it is not illegal. And while I note you do say "if," your statement that the suggestions aren't realistic implies that you seem to think that it is illegal.
Well, I'm not an expert on abbatoirs, I had a quick google, and found a page on my country's government website that says:
"The welfare of animals at the time of slaughter or killing is covered by EU Directive 93/119 and UK regulations. These rules state that it is an offence to cause or permit an animal to suffer avoidable excitement, pain or suffering. Animals must be handled, stunned and killed using specific methods by licensed slaughtermen."
It looks like you're right in that this doesn't mean they have to be anaesthetised as such, as long as they're killed as painlessly as possible. Still, cutting corners so the animals suffer as they're killed seems like it'd be illegal, as would letting staff abuse the animals. There's exemptions for Kosher and Halal slaughter, but I don't know much about how they work.
If you want full efficiency in terms of calorie production, you'd want to go vertical farming, no more creating crops fro animals, no animals on land that can be used for plants and only have animals on land unsuitable for farming plants but that has enough vegetation to support animals.
Yup, there's a good environmental rationale for cutting down on meat consumption, and it's healthier too. Personally I've been trying to switch over to vegetarian meals or fake meat like Quorn for purely health reasons. In terms of efficiency, I've heard that insect protein is particularly good.
In terms of animal welfare though... I bet that if everyone went vegan then most livestock would just die. Some species might even become entirely extinct or endangered. There aren't many people who'd want a full-sized adult pig as a pet and I can't think of any pig products that don't involve killing it. Without being useful for humans, we wouldn't support them, and I don't think many farm pigs would survive if we just released the lot of them into the wild.