You raise a lot of good questions. My concern is that these are busy people. Like most of us, we can assume they get tons of e-mails everyday. Who has the time and/or inclination to respond to a lengthy survey questionaire?
We should reproduce the statements on the 400 list as published by DI and ask if they agree with it, (the e-mail should include a link to the actual list). We should also ask if they agree to the use of their name by DI. We could ask for any additional comments and, IMO, that's about it.
From my experience, I usually don't respond to questionaires. But when I do, I'll answer the first few questions in a short questionaire. If it's a long questionaire, I'll skip it altogether regardless of how interesting I find the subject matter.
The other reason for keeping it short is that some folks might be concerned that their answers on religion, background, etc as part of a lengthy questionaire could be twisted and misused. These folks will not respond.