Is it a sin to avoid sitting or speaking with people you do not feel affection for, and is greeting them and asking about their well-being when compelled to do so considered hypocrisy?

There is no harm in not sitting with someone you do not wish to, and your greeting him and smiling in his face, despite your dislike for him, is not hypocrisy. Rather, it is a matter of good character and مداراة (prudence/courtesy). Al-Bukhari titled a chapter "The Chapter of Mudarat with People" and mentioned from Abu al-Darda' his saying: "Indeed, we smile in the faces of some people, though our hearts curse them." Aisha, may Allah be pleased with her, clarified that the Prophet, peace and blessings be upon him, softened his words to a man who sought permission to enter, even though he had said about him: "He is the worst son of his clan." He explained this by saying: "The worst of people in station with Allah is he whom people abandon or leave because of the fear of his obscenity." Mudarat is recommended; it involves lowering one's wing (being humble), speaking gently, and avoiding harshness. It differs from the forbidden مداهنة (flattery/connivance) which means associating with a dissolute person and showing approval of his actions without denouncing them. As for praising a dissolute person, it is a lie and is not permissible except out of necessity or coercion. You should denounce it with your heart and tongue if you are able, otherwise with your heart and by remaining silent.