If we choose to do something we know is harmful to ourselves, like drinking diet sodas or snacking on processed foods, does that preclude getting divine protection and healing help for the consequences, because it was an act of our free will to choose things that are self-destructive?

We do not judge people and therefore we do not make arbitrary rulings to watch people suffer if they go against our advice or against common sense. It is a common human trait, that human weakness will cause people to bend the rules, take chances, engage in risky behavior because there is some tangible reward for them. There are times when people make bad choices where they truly want a negative outcome, and that puts handcuffs on us, in some cases, to let their decision hold sway. But what you are describing is someone, in effect, cheating to get a benefit but always with some trepidation about what could happen, so their intention is a mixed one, and that will usually give us enough to work with to help support a person who strays from best practices and puts their health at risk, for example, by indulging in a guilty pleasure. But keep in mind that the fact we might still be willing and able to help does not mean we can guarantee a rescue from the consequences they are taking on, as divine healing will be slow, and people have no way of knowing how doing something potentially harmful might add the straw that breaks the camel's back if there is much negative karma aligned with the transgression that is first considered and then indulged in. So we cannot guarantee taking a risk will be overridden by us and made harmless.