An important note: You can decline your exit interview. Unless this requirement is sewn into your signed employment contract, politely decline the request from HR.
Yea, declining is a much better idea. Though in the long term, ideally employees should be able to post these things publicly, which is even better.
I agree. Ideally, that would be nice, but it isn't going to happen. IMO, the most professional approach when you are leaving is to give individuals specific feedback that you think will help them succeed. However, you should already be giving that kind of feedback to individuals, waiting till you are leaving doesn't come off as very helpful or genuine.
Yea, I know that sometimes it is easier said than done.