NPIP was set up by the Feds to combat Pullorum. It's a government program that worked. At one time Pullorum threatened the US poultry industry. Now many states are Pullorum-free. 48 of the 50 states participate in the NPIP program.
Each state is different. Some only require Pullorum testing, others require more. And they have different procedures. I looked at Minnesota's online site. It's got so much information it's confusing. What I know about Arkansas or Louisiana's programs doesn't mean anything to Minnesota.
I suggest you call someone with the program and talk to them. If you follow this link you can get some phone numbers.
https://www.bah.state.mn.us/contact-us/
I'll try to respond to your questions but remember, this is generic, does not specifically apply to Minnesota.
What are the benefits of being NPIP certified?
It's generally requited to ship chickens between states. The requirements of the state they are going to are the important requirements, not the one they are leaving. Some states have internal requirements, some don't. If you want to show them most shoe=ws require NPIP but many shows also have additional requirements.
Does someone come and test your flock regularly for diseases like salmonella, mareks, MG, etc. and then treat/euthanize? If so, how often?
Each state does it differently but they only test for what they are supposed to test for. Some may test the entire flock some only test 10% or something like that. The ones I'm familiar with do not treat or euthanize the birds, though some may require that if certain diseases are found. That generally has nothing to do with NPIP.
In Arkansas you can go to a class and become a certified NPIP tester. Then you can test your own birds and your neighbor's. I have no idea how Minnesota does that.
How are birds that are NPIP safer than those that aren't?
They have been tested for certain diseases so they don't have those.
When sold, are they completely disease free?
Absolutely not. It depends on what they test for. If they test for bird flu you are covered with that. If they don't, who knows?
Are there strict rules regarding spacing and conditions?
Not in the states I'm familiar with.
How are people able to free range with NPIP certification, or is that not a possibility because of the biosecurity risk?
Good question. Ask Minnesota. In Arkansas you can.
How are these strict standards enforced without 24 hour surveillance of some sort?
It's generally the honor system. You agree to abide by certain rules to maintain certification, especially about bringing in new birds. If you break those rules your certification is invalid if you get caught, but they typically don't have people checking up on you. They do have retest requirements every so often.
This is not a police state requirement intended to find a reason to throw you in jail or confiscate your stuff. They are trying to help you keep your flock safe and try to reduce your chances of spreading diseases. Commercial operations probably have minimum legal requirements, especially of you ship across state lines. It's often required if you show chickens. For us backyard flock owners not dong this commercially it's voluntary. Voluntary. You don't have to do it. But it gives some people peace of mind.