Is it obligatory to denounce every sin with the tongue, hand, or heart to avoid falling into the sin of being content with sin, or is there some flexibility in the matter such that the sin is not incurred directly, especially if the one committing the sin is excused?

One should beware of intrusive thoughts (waswas) and not give in to them. Mixing with people and teaching them is better than isolating oneself, unless a person fears being influenced by evil and is unable to change it, in which case isolation would be better. One must change evil with the hand first, then with the tongue, and if unable, then with the heart; for disavowing with the heart is the weakest form of faith, and it is inconceivable to be unable to do so. If you are unable to change evil with your tongue, there is no sin upon you as long as you disavow it with your heart, but you must part ways with the one committing the evil and avoid sitting with them if there is no necessity for it and you are unable to denounce their action.