It is like asking "this glass is 50% full of water. What is the probability that it is half empty? What is the probability that it is half full?"

Both are 100%. Duh.

Likewise, I interpreted MWI to include any interpretation that makes the same predictions as it. I still gave an answer less than 100% because I wouldn't completely rule out all objective collapse theories just yet.

I was especially bothered by the MWI question because it asked whether it was "more or less" correct. Of course it's more or less correct since its math works! But since I assumed the question was intended to find out whether or not I thought favorably of the theory, I just skipped it.

For some of the other probability questions, my answer is "I don't have enough information to come up with a good estimate, and I also don't have enough information to come up with a probability that takes into account my inability to come up with a good estimate". Again, I put 0.

Um, nothing complicated is required here. Consult your gut feeling and put whatever number comes up. Turning this into an actual probability is why we have calibration questions. Putting 0 is definitely a wrong way of refusing to answer the question (you can already leave the box blank, if you really want, and thereby avoid polluting other results).

As an aside, given we had at least three people with such strange interpretations of the MWI question so far, perhaps the survey should include a question asking one's opinion on map-vs-territory...

Also, after the test, I'm starting to get worried how you anonymize the questions. Releasing the data without a name attached is not anonymization, if the answers people give are enough to identify them.

I believe that effective anonymization (apart from removing names) is in general impossible. Although, for specific analyses a reduced dataset can be effective, such as all pairwise joint histograms, which I think preserves a certain amount of anonymity depending how unique each person's answers are.

I also put 0 for MWI, although I feel pretty good about that. (For reasons explained in this comment, a 0 means that my answer is less than 0.5%.)

I am the kind of Bayesian who strictly speaking only speaks of probabilities of potentially observable events. (This is a kind of logical-positivist Bayesianism, I guess.) It doesn't do to be too strict about this sort of thing (I don't want to just wall off entire subjects as unspeakable, which is the classic failure mode of logical positivism), but it does mean that I have to think about what other statements really mean in practical terms.

So I interpreted this to mean, assuming that I learn much much more about the nature of the world than I know now, would I think that the MWI is a useful way for people today to think about things? (That's pretty much how I always interpret questions about interpretations.) And no matter how much learning I contemplate, the log-odds are never as good as 8 bits against, so that's a 0.

Putting 0 is misleading. It implies that you're confident there is no chance at all. If you're really not leaning one way or another, your best bet is to just put 50, or perhaps even skip the question if you really don't want to give a probability.