...if you were a hard determinist/materialist, ethics would be a completely moot issue because everything that happens is the only thing that could have happened. Therefore your own manipulation of said other was already a predetermined phenomenon set in motion by pre-existing material conditions.
You're right here -- this is precisely what I mean when I say the manipulation of others isn't a moral question to the determinist. At which point, the concept of manipulation can be explained by personal motives (pragmatism/utilitarianism).
it's not magic, the other person has a say. if you were really hungry, and someone offered you a sandwich at their house and you went to their house, has your free will been challenged? i guess it's kinda similar to being able to tell when people are hungry and what they want to eat.
You're also right. Free will can't be challenged in retrospect, because it is logically possible for you to have made the choice to stop by your friend's place and have a sammy, as opposed to being impelled to go because you are hungry, you have plenty of gas, and his house has nice lighting, etc.. Any belief in anything is ultimately a matter of faith -- whether it comes to empiricism or determinism or even pragmatism. I readily admit that. At any rate, it's clear that you believe in free will. Ergo, the principle question is whether it is moral to manipulate or influence others. Well, since you believe in free will, it's not like you can ROB someone of free will (because that is a determinist viewpoint). Ergo, it should not be logically possible for you to manipulate anyone anyway. It is possible for you to influence them, but the decision they make after an influence is their own to make.