Self control may be contagious

Neither are your "true preferences" - but preferences changes lead to self-defeating behaviour. If you put your vodka in the loft in the morning to prevent yourself drinking it, and then get it down again in the evening when you change your mind, that's two trips into the loft whose only purpose is to counter each other, which is wasted effort by any measure. Whatever you value, it has to make sense to "straighten out" your preferences so you're not fighting against yourself.

Also, I really do mean "short-term" vs "long-term"; it's not a proxy for "fun" vs "praiseworthy". I had a friend who was damaging her health by taking too many class A drugs. One of the things I tried to say to talk her out of it was that since she liked class As, her goal should be to maximize the total amount of class As she took across her whole life. As it is, her health is now so bad she's had to cut them out altogether, whereas more care when younger could have meant a lifetime of enjoying them.

It could be, in any of those cases. You really don't give enough information.

In the first two cases, it could be willpower, or habit and/or inertia, or some combination of the three. In the third, it depends on whether you find the chips to be tempting or not - making the same kind of decision over whether or not to buy apples is significantly less likely to involve willpower. In the fourth case, it could go either way, depending on how you react to that kind of situation: I find the rewards of that kind of assertiveness obvious enough that doesn't usually take willpower to be assertive, but I also know people who find that kind of situation stressful and would definitely have to make an effort to stick to their decision.

This definition may be useful (though I usually like to mull definitions over before I share them, which I haven't in this case): Willpower is the ability to override your system one reactions and impulses so that you can accomplish something that doesn't have a reward that's visible to system one. This implies that your system-one's ability to model things (edit: and not get distracted by shiny things or overwhelmed by emotion) will have a large effect on what things you need to use willpower to do. (Which explains why I'm not a fan of using willpower, too.)