Can Jelava cast a creature if it has Flash?
No, you cannot.  Being able to be cast when an instant could be cast does not make something an instant.  The closest the rules actually come to addressing this point appears to be (emphasis added):


  304.5. If text states that a player may do something "any time he or she could cast an instant," it means only that the player must have priority. The player doesn't need to have an instant he or she could actually cast. Effects that would prevent that player from casting a spell or casting an instant don't affect the player's capability to perform that action (unless the action is actually casting a spell or casting an instant).


Thus, "having flash" does not imply "being an instant."