If Jedi Training is a big deal, definitely stand (with child) in line at HS 45 minutes prior to opening. Then walk as fast as you can when the park opens back to JTA signups (child must be with you). You should be done by 9am, plenty of time to head over to breakfast.
Go to youtube and show your child the video. The show is the same/same choreography/instructions. You can prep your child for the show so that they know what to expect. No way my oldest would have wanted to participate...probably not until he was 6. Now, well my 4yo might have conquered DV last January when he was still a couple months shy of 4. He was awesome (knew he would be), and the certificate from JTA is one of his most prized posessions.
As far as the questions...they are really just making sure they are the correct age and that they are capable of following instructions/answering questions. In my nearly 4yo case, he stated he was 4, then my 8yo said 'no, he's 3' then the CM smiled and said Happy Birthday. They do enforce the rules, but aren't aggressive about them/looking to exclude someone who could participate.
Besides the size of DV, the only other part that can be tricky is opening/closing the light sabers. If you have one at home, get him used to pushing button/throwing it up (or use gravity), and then closing it up. The 'Jedi Trainers' are great about helping them out, and always have little quips to make the audience laugh about it.
They can also tie/knot the corners/edges of the robe they wear so that they don't trip on them. Pretty sure it is a standard practice, but if your CMs don't do that, it is a good idea.
Happy Birthday to your LO!