I will agree with you in your example, that replacing กวนตีน with ล้อเล่น could come off more respectful if you're not sure about offending someone. But you make it sound like มัน มึง กู and กวนตีน are bad words and should not be used unless you 'are a master of the Thai language' which I strongly disagree with. These words are simply more intimant and informal, not bad or generally impolite---only sounding impolite when crossed with formality, which certain social masks are expected to be worn ครับ.