You don't sound like you're going to have a very profitable business LOL.

Breeding rodents as a business is in general a pretty cut throat enterprise. People will f' you over any chance they get. Neighbors will complain if the catch wind of what is happening. The animal rights blow hards will be busting your stones.

Every question that you just asked can be answered in one place. In your town, if rats are considered a nuisance species, do as you wish. That's it.

It would take a Vet a month of non stop working to inspect a rodent facility rat by rat. It can't happen.

Zoning laws are different town to town. My guess is that if you are going to be selling them out of the building that they are being bred in then you would need to have it zoned as a business. If not, then you can do as you like.

The USDA is the department of agriculture. They usually govern things that are eaten by people. That is up to you. Some states or towns require it. Some don't.

Everything will be up to the town that you open up shop in. I would personally choose to open somewhere without all of the red tape.