I am very sorry to hear that the interview you had lined-up got canceled. I do not know if my answers may help, but since I've been subjected to "leading questions" myself, I'll throw in my two cents. (Just to give you some background, I am a forensic scientist and I'm often called to testify in court as an expert witness.) If you are able to interview somebody who is more suitable, by all means, please disregard my comments.
1. What is your definition of "leading questions"
In my experience, leading questions are typically those that are asked by an attorney, which direct the witness towards the desired response - usually only allowing for a "yes" or "no" answer. Under certain rules, witnesses providing what is called "direct testimony" are not allowed to be asked a leading question. On cross-examination, however, or if the witness is deemed to be "hostile", then the attorney is allowed to ask leading questions in order to elicit the information he or she desires. Although television and movies are not always terribly accurate, you will probably have seen attorneys using lines such as, "Isn't it true...?" That is an example of a leading question, since the attorney is pretty much telling you to agree (or disagree, as the case may be).
2. Do you think the ability to have questions that lead, important or unimportant in today's society
Based only on my experience in court, I would say that it is important. However, using a similar line of questioning outside of the courtroom might appear rather odd. For example, "Isn't it true that you would like pasta for dinner?" Sounds weird, doesn't it?
3. In what jobs might this kind of skill be usefull?
Definitely for a career in law, as discussed above. However, I can see when it would be a useful skill as a journalist or an interviewer.
4. How do you know about leading questions?
Personal experience from testifying in court.
5. What uses do leading questions have?
Again, going back to court, they allow for an attorney to control the line of questioning and to limit the responses from the witness. In effect, this flips the roles of the examiner and witness, whereby the attorney is the one essentially explaining what happened, rather than the witness. There are limitations, however, since it does not allow for a narrative from the witness. Thankfully, there are exceptions for expert testimony!
I hope these at least allow you to include something in your report. I'm sorry if these were not the sorts of answers you were looking for. Please be sure to let me know if you have any more questions, or would like me to expand further on anything.
Firearm & Toolmark Section
Ventura County Sheriff's Department
Forensic Sciences Laboratory