If you can't tell the difference, I think most people would assume that the one being nice to them liked them, since that's the most pleasant thing to believe. And if you know that the person you're being nice to is going to take it that way, then aren't you lying?
The nearest I can figure, it rather depends on your intent in maintaining polite interaction.
I am typically easy-going, polite, and friendly. Well, more accurately, people would typically describe me as being that way. I treat everyone that way as a matter of conscious choice. I have no responsibility for their interpretation of my behavior, do I? Therefore, I'm not being deceptive because I am just being me.
On the other hand, if I were to butter someone up by behaving more friendly than is typical in order to achieve some purpose, then I'm practicing a shameful deception.
All of which doesn't answer your question, exactly. I'm not very good at situational ethics.
Love your enemies is practical advise on an esoteric level. After all, if you are kind to someone who dislikes you, he or she may change his or her mind about not liking you. If you are true to your consciously good nature, you will not have reason to feel guilty over your actions. And so on.
I have known many people, whom I could not stand, who thought I positively adored them. Is it my responsibility to correct them? Would it hurt them to know they are wrong?
By pretend to like someone, do you mean actually indicating (unequivocally) that you do like them or do you mean behaving civilly and politely. I should think that makes all the difference to your answer.
I really can't actually answer this question because if there is one thing that genuinely confuses me it's situational ethics. I tend to follow black and white rules.