how do you know that you can trust the action of the residents of the "Executive Mansion" and Congress?

Answer 1

You don't. The best way to ensure that the various branches of government are going to be honest is to elect people who may not necessarily be honest themselves, but who will be vigilant against the other branches.

A good example is with Bush - had Congress actually performed meaningful oversight, and had they seriously challenged the administration when the administration was being obviously dishonest, vague, or obstructionist, then perhaps a lot of the dissatistfaction with Bush and the government in general would be muted - the public would see that the political process was actually working in the manner intended.