I think this example shows that what matters is not the consequences of your actions, but your intent when you take those actions.

From whose point of view ? If you are committed to poisoning your hapless friend, then presumably you either don't care about morality, or you'd determined that this action would be sufficiently moral. If, on the other hand, I am attempting to evaluate the morality of your actions, then I can only evaluate the actions you did, in fact, perform (because I can't read your mind). Thus, if you gave your friend a cup of tea with sugar in it, and, after he drank it, you refrained from exclaiming "This cannot be ! So much cyanide would kill any normal man !" -- then I would conclude that you're just a nice guy who gives sugared tea to people.

I do agree with you that intent matters in the opposite case; this is how we can differentiate murder from manslaughter.

I would say this is the light of truth shattering your illusion about being a good person then. Maybe that realization will drive you to overcome the akrasia...

Maybe it won't, though. Thus, we have traded some harmless delusions of goodness for a markedly reduced expected value of my actions in the future (I might still do good deeds, but the probability of this happening is lower). Did society really win anything ?

If I wanted to go to X less, then my friend would want to go to X less.

Sounds like this is still mind control, just to a (much) lesser degree. Instead of altering your friend's preferences directly, you're exploiting your knowledge of his preference table, but the principle is the same. You could've just as easily said, "I know that my friend wants to avoid pain, so if I threaten him with pain unless he goes to X less, then he'd want to go to X less".

I can try again with another hypothetical. A girl wants to try ecstasy...

I don't think this scenario is entirely analogous either, though it's much closer. In this example, there was a very high probability that the girl sustained severe lasting damage (STDs, pregnancy, bruising, drug overdose or allergy, etc.). Less importantly, the girl received some misleading information about drugs, which may cause her to make harmful decisions in the future. Even if none of these things happened in this specific case, the probability of them happening is relatively high. Thus, we would not want to live in a society where acting like the drug dealer did is considered moral.