While I feel like you could maybe detonate one on land, I’m not sure you could “fire” a missile without total control of all sorts of infrastructure, like all sorts of networking, crypto, spy satellites, and maybe even troops in each silo. Even then I think that the early warning systems (afaik they can know about a launch but not necessarily the target) of other nation states would make it a risk to fire one even within the US because it would risk retaliation and mutually assured destruction

You shouldn’t worry about US balkanization as a real possibility. The US even with its cultural divisions is still one of the most unified countries on the planet (it’s probably one of the least likely countries in the American continent), especially among the bigger ones to balkanize Canada and Mexico are certainly more likely to balkanize first. It’s also an order of magnitude more likely that you’d see balkanization of India, China, Pakistan or something first (other nuclear powers are similarly unified in the current year to a comparable degree as the US).

The biggest cause of balkanization is linguistic divisions, especially when their combined with other things like religious or broader ethnic division, and preexisting strong levels of local autonomy that tend to exist in a lot of other countries far more than they do in the US, especially as it relates to cultural education and development.