Why would you write a letter if you don't know what you're objecting to or even if you should be objecting?
The location is classified.
Ok all jokes aside, if you suspect that there’s wrongdoing in the classified sphere, and it really matters to you, well, you should get involved in politics. We don’t just let everyone everywhere know everything, because we think it would be risky if Putin or the Chinese Communist Party also knew all those things. So we limit it to people who have taken oaths and are accountable and need to know (the military), the civilians who need to know (security clearance holders), and those who hold a high office with the public’s trust (high-ranking politicians). You can be a Senator. You just need a lot of people to trust you enough to vote for you. Or, and this is a bit easier, support politicians you do trust to vet classified things to be elected to high office, and ask them to look into it and give you their word that things are being done properly.
This is why we elect competent (hopefully) leaders to worry about these things for us. Mob rule democracy about every national secret would mean they’re not secrets for very long!
Surely you are responsible for the consequences of what you do, no matter how indirect? After all, we live in physical reality, not in some world of laws.
If you cause something you cause that thing. You are reponsible, even if it is through some long chain.
No, that’s preposterous. You are not responsible for the actions of others simply because your actions put them in a place to perform said actions. That seems like a very stressful way to go through life.