Some stuff out there is pure tinfoil hat stuff, other stuff is real but exaggerated.
I've seen my chickens eat mice. Meat per say is not bad, they are omnivores just like us, however, I doubt those mice ingested arsenic, antibiotics, grown hormones, or chicken feces. I wouldn't raise mice on those things and then feed them to my chickens. Sometimes wild does not mean worse.
Yes, rendering plants do turn animals that drop dead into animal feed. I can't comment on pounds (ie cats and dogs). Watch Dirty Jobs season 5 episode 14 where you will see how they do it. Its very educational, but disturbing. There are clips on you tube I believe.
Our industrial food production system wastes nothing that can be turned into a product. Its not a shocking secret, people just normally do not choose to look behind the curtain about such things.
By-products can contain anything. It can include parts from beef, pork, turkey, meat chickens and battery hens from egg factories, fish parts, organs, blood, bone. Even chicken poop is used in feed.
http://www.huffingtonpost.com/david-kirby/you-want-chicken-poop-wit_b_530404.html
Some things are used for human food too. A common additive to ground beef is a product made from the outer fat layer that is exposed after skinning. Because it comes from the outer exposed surface of the animal it needs to be treated with ammonia solvents to retard bacterial growth in it. Then it is ground into paste and added back to processed meat. Its been in our diets for many years, even our schools USDA meat but most are not aware of that.
http://www.organicconsumers.org/articles/article_19947.cfm
This all sounds shocking and crazy and some over react when talking about it but its not tin foil hat stuff, its business as usual. Its just the way the National Food Industry is, they have to find a use for the waste or its money down the drain for them.
If you have immediate problems you can always work your way little by little towards an alternative. Supplement with the feed and try different things until you find something that works for you then wean your self onto your independent food source.
Have you considered duckweed? Its easy to grow in a backyard kiddy pool and its about 40% digestible protein.
Don't forget that commercial feed didn't exist 100 years ago yet people kept chickens just fine. Keep trying.