> Only completely failing #0 (i.e. having no clue at all) would disqualify a candidate, IIRC.

It is good, but the other points of interest have their influence on the later decision, isn't it?

> "yes, we see that you understand this and it is a toy problem, please humour us."

This way you are showing that you have some hidden goals, it means that situation is really uncertain. In such a conditions anyone would feel stress and anxiety. Maybe except those who are not interested in outcomes of interview or those who have psychological disabilities and is unable to feel anxiety at all.

Lets assume a young candidate, who all his previous life spent on different exams and tests, and therefore percieve this interview as something like exam.

> 0. You are capable of solving this 1.

Ok. I believe that this is a non zero amount of information.

> 1. You can take an educated guess at the complexity

If he start to talk about this, than you would be able to measure his ability to do this, but probably he wouldn't start to talk, due to fear to make a mistake. Like on exam, where it is good to show what you know, but not good to show what you do not know.

> 2. You are capable of speaking out if the task seems odd.

No. He probably would not do that. One of the strategy on exam is to present your best guess with as much confidence as you can express.

It is just one of the possible discourses through which candidate could see this situation. I'm not sure which are other possible discourses -- to figure them out one would need to watch a lot of interviews and to ask odd questions to candidates, to explore deeper than one can judge about person discourse based on behaviour only. But the point is: if you confuse a human by situation, the human would experience a stress, and would start to behave in strage ways, which are led not by his/her personality, but by stress itself. And the more unfamiliar overall setup for the person, the more strange ways to behave you would see, because person would find something in his/her past which resembles this sutiation most closely (exam, test, initiation procedure in the informal group of teenagers in the college, or anything like) and would behave accordingly (without conscious decision of course).
If interviewer does not take into account the actual discourse candidate is using, if interviewer is using his own discourse to make meaning of behaviour of candidate, that he fools himself, seeing not the real personality of candidate, but something else. It is highly probable that he would see something completely wrong.

If you wish to use fizzbuzz for #0 and #1, than do not ask candidate to solve task. Ask him what the first thought he had when sow this task. Ask ten more questions. Break all his discourses and make a new one. Create a discourse of discussing and collaborating, something that resembles real work atmosphere. It would be difficult if person is already in the stress and therefore tighly bound to some other discourse, but if you fail to do this, than all your behavioural tests are not valid. At best such an interview would show technical skills of candidate (if he is capable to use them under the high stress), but does not show his/her psychological/social traits.