Since it differs from his own universe's psychic abilities, am I right in assuming that Zexion's magic (excluding once it's been cast as an elemental spell like Fire or Blizzard) & Illusion-based abilities will require experiencing them first, for Accelerator to use Reverse-Engineering?

I am canonblind for KH so I will go out on a limb and say prrrrrobably. Canon doesn't give me a lot of examples of how magic used on/against him reacts to his ability. Elemental spells like Fire & Blizzard do get partially redirected and dissipate at an angle, though; someone used a water attack on him once and that's what happened, complete with rainbow. Illusion-based stuff is more in that grey area - sounds he should be able to block without experiencing first (assuming he knows to block it), but most other fooling of his senses not so much. In the event that Zexion does use any illusions on him we can discuss the particulars in more detail, just for the time being it's safe to assume that for the most part, it is as you assume.