a/ There are connected military computers which are important. That assumption is based on the people who have been charged for hacking into such systems already.
Not really.
Most of the time when people hack government or military computers, it's the public webpages and the like. Essential data is almost never compromised.
b/ That "accidental" or illicit connections can be made that connect systems that are not supposed to be connected. By illicit I mean unauthorized not intentionally criminal.
True, this does indeed happen from time to time. However, the nature of the networks makes it very difficult for one or a small group of computers to disrupt major portions of the network.
c/ That people make mistakes and transfer infected data between isolated systems.
This happens too, but as I said, the nature of the networks tends to isolate disruptive elements on the network.
I'm not saying that military and government networks are invulnerable...I'm only stating that they are FAR more robust to attack than the civil internet.