When has there ever been due process or checks and balances in time of war?
AlpQuaeda is at war with the USA. That is just a fact. We can ignore them or go to war with them. But if we go to war, by definition there won’t be anything resembling due process.
I understand your concerns and this is indeed a slippery slope. But when it is necessary to cross such a slope to defend American lives, you do so, and protest when excesses are proven, not because excesses CAN happen. If that tack is taken, there can be no effective self-defense.
Checks and balances ALWAYS operate even in times of war. And due process usually operates as well. That is why our courts continue to operate even during war time. Our Congress continues to sit even in war time. And the Supreme Court has declared that our right to due process continues during war time. (See Ex Parte Milligan)
The Constitution cannot be suspended even in time of War. Consequently checks and balances continue to operate even in time of war. This is both true historically and theoretically. The only time that the normal procedures of due process can be ignored is if there is something like actual combat in the streets that make it impossible to operate the courts. (Again see Ex Parte Milligan).
Congress has to suspend Habeas Corpus to limit due process. It doesn't happen just because Congress declares war. Congress has to specifically suspend it, and Congress can only do this if the the safety of the public requires it.
But when it is necessary to cross such a slope to defend American lives, you do so, and protest when excesses are proven, not because excesses CAN happen.
How could you prove there are any excesses? The executive will just classify the information and deny that a crime occurred. Also, what is to stop the government from accusing any whistle blowers of being a threat and getting rid of them too? The system that you are proposing is more than just a slippery slope. It is near the bottom of the slippery slope. The system that you are proposing is blatantly unconstitutional. Under the system your post advocate, you only have rights when the government says you can.
Is it excessive when a 16 year old American citizen is murdered by drone? BTW, the 16 year old was wanted by nobody, been accused of nothing and never had a day in court. Is it OK for the king to murder him because you fear Al Qaeda?