There are entire political industries openly dedicated to swaying the undecided! It's a messy business, but that's what we have.

Propaganda is not necessarily to gain influence or money. Eg: Country x just wants to mess with people's heads and turn them on each other to weaken a rival country. Or: Country y runs a crafted propaganda campaign against a rival. As a result, some sector of its own economy starts doing better at the expense of its rival.

>If some internal "threat actor" gets control of the executive branch and of the media and also can prevent information flow from the outside, very little can be done against it.

I understand the scenario (it's far from new), but that's what the design of any particular democracy is supposed to minimise. Term limits, separation of government powers, etc.