This doesn’t mean that all is pre-determined and that if we all stop trying we’ll turn out the same we would have otherwise, but rather that we are playing within certain parameters, and that the part we control is probably smaller than most people think (not non-existent — we still deserve some credit — just more modest).

A lot of these concepts (willpower, choice, responsibility, blame, credit) really start to break down if you look at them too closely, and this post is using them a little too formally for my comfort. What do you actually mean by "the part we control", or "if we stop trying"? This only makes sense if we all agree upon some set of counterfactuals, some set of events that "could have gone differently".

People do what they do. If you can predict what that is (genetically or otherwise), then you can predict it. If you can't, then you can't. To reify this difference into some intrinsic property of minds like "free will" is to commit the mind projection fallacy.

You've singled out genetics and seem to be implying that people shouldn't be blamed for a "lack of willpower" if that lack has a genetic origin, but any such lack must have some cause, so why are genes so special? If parental behavior has a significant effect on apparent willpower, is the individual suddenly "blameworthy", or do we start down the infinite regress of blaming their parents?

What if the person in question takes a prescription medication and suffers a side effect that diminishes their willpower, are they to blame for that? ...If the side effect was unknown? ...If the side effect was known to be extremely rare? ...If the side effect was known to be fairly common, but the person's perceived need was great enough to take the risk?

To be clear, I’m not saying the situation was white and we thought it was black, or even that it’s a black & white thing, but rather that most people’s intuition might be the wrong shade of gray.

If "most people's intuition" is founded on an incoherent notion of free will, then I don't think minor tweaks to the shade of their delusions will help them get any closer to the truth. Our intuition of blame has more to do with what we can imagine intentionally changing (parental behavior, environments) than any fundamentally meaningful distinction. As gene therapy becomes more effective and accessible, this line is only going to get blurrier.