How can we even start defining CEV without brain scanning technology able to do much more than answering the original question?

It would seem that we can define the algorithm which can be used to manipulate and process a given input of loosely defined inconsistent preferences. This would seem to be a necessary thing to do before any actual brain scanning becomes involved.

Well part of my point is that indeed we can't even define CEV today, let alone solve it, and so a lot of conclusions/propositions people put forward about what CEV's output would be like are completely unsupported by evidence; they are mere wishful thinking.

More on-topic: today you have humans as black boxes, but you can still measure what they value, by 1) offering them concrete tradeoffs and measuring behavior and 2) asking them.

Tomorrow, suppose your new brain scanning tech allows you to perfectly understand how brains work. You can now explain how these values are implemented. But they are the same values you observed earlier. So the only new knowledge relevant to CEV would be that you might derive how people would behave in a hypothetical situation, without actually putting them in that situation (because that might be unethical or expensive).

Now, suppose someone expresses a value that you think they are merely "lying, trolling or joking" about. In all of their behavior throughout their lives, and in their own words today, they honestly have this value. But your brain scanner shows that in some hypothetical situation, they would behave consistently with valuing this value less.

By construction, since you couldn't derive this knowledge from their life histories (already known without a brain scanner), these are situations they have (almost) never been in. (And therefore they aren't likely to be in them in the future, either.)

So why do you effectively say that for purposes of CEV, their behavior in such counterfactual situations is "their true values", while their behavior in the real, common situations throughout their lives isn't? Yes, humans might be placed in totally novel situations which can cause them to reconsider their values; because humans have conflicting values, and non-explicit values (but rather behaviors responding to situations), and no truly top-level goals (so that all values may change). But you could just as easily say that there are probably situations in which you could be placed so that you would come to value their values more.

Your approach places people in the unfortunate position where they might live their whole lives believing in a value, and fighting for it, and then you (or the CEV AI) come up to them and says: I'm going to destroy everything you've valued so far. Not because of objective ethics or decree of God or majority vote or anything objective and external. But because they themselves actually "really" prefer completely different values even though on the conscious level, no matter how long they might think and talk and read about it, they would never reach that conclusion.