A. The startle results are match the second and third shots, as explained in Why Jackie Scowled. It would be great to have the time and money to attempt an accurate duplication, but it is not required: one of the key attributes of science is its ability to predict and explain complex events even when the exact circumstances can’t (yet) be reproduced.
The exact size of startle reactions are subject to the experience, posture, excitement, age, condition, anticipation, stance, and interest of the subject. If the subject has trained extensively with firearms then often only their eyes will blink. Startles are greater when the subject is standing up and already excited. In Zappruder's case, he was so intent on capturing an historic event that he kept his camera rolling despite possible dangers - though he did let the aim point wander as he looked around while filming.
Accurate reproduction is likely too expensive, because the subject should be: