A county is not a sovereign entity. It is a municipal corporation subject to the pleasure of the state in which it is incorporating. A state is sovereign. The federal government is, but it is extremely limited by the constitution. Unfortunately because we exist under Common Law too much fraud was allowed to be introduced into the system. Our current government is in direct violation of the founding documents (AKA the social contract) under which it was formed.

Both of these ideas of State sovereignty are frauds perpetrated on the people. The people are sovereign. We are led to believe instead that the government is sovereign and the people are vassals. This is a violation of Natural Law (which is impossible, though not impossible to make people believe) and a fundamental fraud in the construction of The Matrix.

The only sovereignty inherent in a government is the sovereignty implied by the group of sovereign citizens with a joined purpose that make it up.

When a government is founded, it is founded with articles of incorporation (in the case of the original U.S. government that article was the Constitution). This makes it a legal person made up of a group of people. This allows for that group to be a single party in any legal grievance (lawsuit).

In such a system the individual and the government are both the same level of entity. Both sovereign, with the rights inherent in a sovereign (which are actually just our inalienable rights). In the case of a government it will likely have fewer rights than the citizen, but it can't have more. It only has the rights necessary for it to carry out its purpose (a common defense e.g.). Even within those rights it never has the right to violate a citizens rights. The fifth amendment was a direct violation of citizens inalienable rights. So were several after that. The fraud was there from the beginning.

The real fraud comes in when the Corporation becomes a separate entity from the sovereign members that make it up. I am not sure when that happened (in a legal sense). It may have been there for quite a while. In the case of the U.S. it may have been there from the beginning. I haven't quite worked that out yet.

Perhaps if Kyle is charged by the Federal government this fraud will come to light.

I believe that any local-level crime (such as a county-level violation) would still be considered a state-level prosecution.

However, I am not a lawyer and do not know this for certain, nor do I have enough time at the moment to confirm that at the moment. That also largely prevents me from giving you a useful answer to your second question.

I'm not defending the practice or saying I understand the intricacies of it from a legal standpoint, as I do not work in the legal field. I only know that a state and federal charge can be leveled for the same crime from the same event without violating double jeopardy.

To be clear, though, I highly doubt the DOJ will be interested in keeping Rittenhouse in the news at this point. Even if the prosecution hadn't completely fucked up the case, I still find it unlikely they would have won based on the evidence that was presented in court. Unless there is some massive piece of evidence that both the prosecution and defense failed to address, I can't imagine a federal trial would have any reason to come to a different conclusion, and I cannot imagine why the DOJ would want to devote resources to what would likely be a losing case that, as you stated, is under significant accusation of being politically-tinted. Even if they win, they'd likely lose.