Empirically, ethos in the sense that you seem to be using it here simply doesn't persuade - or rather, it persuades only in very rare and exceptional cases like MLK and Gandhi. And what makes them persuasive isn't ethical speech but ethical living. This creates a basis for trust and authority which can't be communicated using rhetorical techniques alone.

Compare this with recent history. Italy, the US, and the UK have all fallen prey to con artists who have no moral stature. And in France, the debate has set yet another con artist against someone who's far more ethically sound, but simply hasn't inspired the voters.

It's tempting to blame a propaganda war, but that ignores the fact that those who don't buy the bullshit are a minority, while those who do buy it - wholesale - are the majority.

So I don't find the concept of ethos useful here, because it starts from the false assumption that everyone's ethos is similar.

If you get into a debate with someone whose ethos is based on power and expediency rather than respect and honesty, you will lose. A liar who appeals to simple-minded and base instincts will always be more persuasive than someone whose position is nuanced and ethically sound.

Proving that the liar is charlatan is probably the only effective strategy. And sometimes even that doesn't seem to work.

So I can't agree that assuming the moral high ground on a basis of personal impeccability has implicit rhetorical value.

What works in practice is being able to persuade others that you have moral authority.  This is completely different to actually having it. And it seems to use a completely different set of rhetorical tricks. (E.g. identifying with god forms and other tribal totems, linking personal power with tribal power and military prowess, assuming historical, scientific, economic, or even mythological inevitably, and so on.)