Re: Okay, so good manners should be interpreted at face value. Thus when one puts 'good manners' on display, one does not claim that he has good motives.
It is not my business what you choose to infer from good manners. Please, infer good motives from good manners if you wish, I often do the same, but do not expect that inference to be true in every instance. The motivation behind the behaviour of another is often a mystery, even where their behaviour is identical to our own at face value.
Re: Etiquette is not means to the end of expressing a benign attitude, but is an end in itself.
Etiquette might be a means of expressing a benign attitude, or an end in itself. In fact, it might be an means to almost any end you might imagine, from bartering a cheaper price to greasing the wheels of communication.
Re: Don't see the relevance. With the two aforementioned maxims it is acceptable (though preferred otherwise) to have a malignant attitude whilst putting good manners on display as the latter is an end in itself and makes no implicit statement in regards to one's inner attitude.
Think for a moment what it would mean if good manners without good motives were unacceptable. Is it preferable that people with bad motives also have bad manners?