Firstly, thank you for replying and spending the time to discuss this with me.

P: Humans naturally or instinctively act according to a system very close to Utilitarianism

Were this true, the utilitarian answers to common moral thought experiments would be seen as intuitive. Instead, we find that a minority of people endorse the utilitarian answers, and they are more likely to endorse those answers the more they rely on abstract thought rather than intuition. It seems that most people are intuitive deontologists.

I admit I made a bit of a leap here, which may not be justified. I was careful to specify 'very close', as I realize it is obviously not an exact copy. I would argue that most people do attempt to follow Bentham's original formulation of seeking pleasure and avoiding pain instinctively, as that is where he derived his theory from. I would argue that though people may implement a deontological system for assigning moral responsibility, they are ultimately using Utilitarian principles as the model for their instinctive morality that describes whether an action is good or bad, much the same as Rule Utilitarianism does. I don't think I can overstate the importance of the fact that Bentham derived the idea of Utilitarianism from a human perspective.

I don't think "nihilist" is an interesting term, because it smuggles in implications that I do not think are useful (like "why don't you just kill yourself, then?").

In the longer formulation, I tackled this exact question, pointing out that is is more effort to overcome your survival instincts than it is to follow them, and thus an illogical attempt to change things which don't matter.

I like 'nihilist' as a term as it is immediately recognizable, short, punchy, and someone with a basic grasp of Latin or maybe even English should be able to derive a rough meaning. It also sounds better. :P

The practical advice I would give: do not seek to use ethics as a foundation, because there is nothing to anchor it on.

Well, as it currently stands, I'm happy with the logical progression necessary to reach my current understanding, and more importantly, it has given me a tremendous sense of inner peace. I don't think that it as such limits my mental progression, since I arrived at these conclusions through rational means, and would give them up if confronted with sufficient logic contrary to my understanding.

If there is no intrinsic value, then let us look for extrinsic value.

Would you mind elaborating on looking for extrinsic value? Is that like the Existentialist viewpoint?