Part of your description of the "ethics is willpower" position appears to be a strawman, as other parts of the same description are accurate, I assume it is because you do not fully understand it:

Firstly the position would more accurately be called "ethics is willpower plus wisdom", but even that doesn't fully capture it. Let's go through your points one by one:

Ethics is specifically about when your desires conflict with the desires of others. Thus, ethics is only concerned with interpersonal relations.

No, it also includes delaying short-term gratification for long term benefits even to yourself.

There is a single, Platonic, correct ethical system for a given X. (X used to vary by social class but not by context or society. Nowadays it is allowed to vary by society and by context but not by social class.)

This is more or less correct.

Your desires and feelings are, if anything, anti-correlated with ethical behavior. Humans are naturally unethical. Being ethical is a continual, lifelong struggle.

No, it's just that when they agree we don't spend much efforts talking about it. This is similar to the reason people on lw focus on the flaws in human reasoning and not the many times it's correct.

The main purpose of ethics is to stop people from doing what they naturally want to do, so "thou shalt not" is more important than "thou shalt".

Well yes and no, there are positive imperatives as well as negative ones.

The key to being ethical is having the willpower not to follow your own utility function.

By "utility function" you seem to mean one's selfish desires and/or impulses.

Social ethics are encouraged by teaching people to "be good", where "good" is the whole social ethical code. Sometimes this is done without explaining what "good" is, since it is considered obvious, or perhaps more convenient to the priesthood to leave it unspecified. (See the Koran for an extreme example.)

No, in fact a large part of the "traditional education" was teaching people how to distinguish good from bad.

The key contrast is between "good" people who will do the moral thing, and "evil" people who do just the opposite. Turning an evil person into a good person can be done by reasoning with them, teaching them willpower, or convincing them they will be punished for being evil.

No problems with these two, except I'd potentially replace the "or" with an "and".

Ethical judgements are different from utility judgements. Utility is a tool of reason, and reason only tells you how to get what you want, whereas ethics tells you what you ought to want (or what you ought to do, whether you want to or not). Therefore utility judgements, and utilitarians, are unethical.

Except it's also possible to reason about what you should want so ethics and reason aren't opposed. Furthermore, once you've figured out what you should want you should also use reason to figure out how to go about it.

Human society requires spiritual guidance and physical force to stop people from using reason to seek their own utility. Religion is necessary even if it is false. Reason must be strictly subordinated to spiritual authority.

I don't entirely agree with these, but they are at least positions that a lot of people actually hold.

Smart people are less moral than dumb people, because reason is a handicap to ethical behavior.

Uhm no, see my note above about it being possible to reason about ethics. Unethical smart people, however, have the potential to be worse then unethical dumb people since they can better rationalize their misbehavior and go about doing it more effectively.

Since ethics are desirable, and yet contrary to human nature (including human reason),

That's like saying that reason is contrary to human nature because we have biases.

they prove that human values transcend logic, biology, and the material world, and derive from a spiritual plane of existence. If there is no God, and no spiritual world, then there is no such thing as good. Sartre: "There can no longer be any good a priori, since there is no infinite and perfect consciousness to think it."

Even the people who hold this position believe it's still possible to reason about it.

You become better at making ethical decisions as you mature, because it requires willpower, which children notably lack; and wisdom, which is sort of the spiritual version of intelligence, except it is non-quantifiable, and is acquired automatically as a linear function of age.

This suggests you don't understand what people mean when they use the word "wisdom". As a first approximation think of wisdom as meaning real world experience.

A person's ethicality is a single dimension. Since ethicality is determined by the degree to which a person has willpower and subsumes their utility to social utility, each person will have a level of ethicality that will be the same in all domains. You can be a good person, an evil person, or somewhere in between - but that's it. You should not expect someone who cheats at cards to be courageous in battle, unless they really enjoy battle.

I suspect this is true to a similar extend that intelligence is a single dimension.